SYSTEMS AND METHODS FOR RUNNING A NOx SELF-DIAGNOSTIC TEST

ABSTRACT

Methods and systems are provided for detecting NOx sensor degradation based on results from a NOx sensor self-diagnostic (SD) test. In one example, a method may comprise: determining that a nitrogen oxide (NOx) sensor is degraded if outputs received from the sensor via a CAN bus during a self-diagnostic test are outside a threshold range. Additionally, only outputs received from the sensor during a self-diagnostic (SD) test performed after a first completed SD test after a key-off event, where the outputs are generated under conditions where a temperature at the sensor is less than a threshold, and an oxygen concentration is within a threshold range may be used to determine if the sensor is degraded.

TECHNICAL FIELD

The present application relates to the control of emissions in vehicleexhaust systems.

BACKGROUND AND SUMMARY

Selective catalytic reduction (SCR) catalysts may be utilized in theexhaust systems of diesel engines to reduce NOx emissions. A reductant,such as urea, may be injected into the exhaust system upstream of theSCR catalyst, and together, the reductant and the SCR catalyst maychemically reduce NOx molecules to nitrogen and water, thereby limitingNOx emissions. However, if a component of the NOx emission controlsystem, such as the SCR catalyst, becomes degraded, NOx emissions mayincrease. NOx sensors, configured to measure NOx levels in the exhaustsystem, may therefore be positioned in the exhaust system to detectfailures of the NOx emission control system. Specifically, increases inNOx levels that may be indicative of degradation of one or morecomponents of the NOx emission control system may be detected by the NOxsensors. Thus, the efficiency of the SCR catalyst, and other componentsof a NOx emission control system may be monitored by one or more NOxsensors positioned in the exhaust system.

NOx sensors may also become degraded, and estimations of the NOx levelsmay become less accurate. As a result, NOx slippage in the exhaustsystem may not be detected. In order to monitor the accuracy of a NOxsensor, the NOx sensor may run self-diagnostic (SD) tests after anengine key-off event where an engine is turned off. One such attempt todetect NOx sensor degradation is described in US Patent Application2014/0144126 to Kowalkowski et al. The disclosure attempts to detect NOxsensor degradation by running one or more SD tests during an engineafter-run state.

However, the inventors of the present application have recognized aproblem with the above solution. In particular, the disclosed attemptdoes not address a significant contributor to the NOx levels estimatedby the NOx sensor. As one example, urea injected into the exhaust systemduring engine operation, and/or urea droplets delivered to the exhaustsystem after the key-off event during a urea delivery line purgingprocess, may persist in the exhaust system after the engine key-offevent. Under sufficiently high exhaust temperatures, urea in the exhaustsystem may be converted to ammonia. NOx sensors register ammonia as NOx,and therefore NOx levels may be overestimated. Therefore, the accuracyof NOx SD tests may decrease with increasing exhaust temperatures andurea concentrations in the exhaust system.

The inventors herein have devised systems and methods for addressing theissues described above. In one example, the issues described above maybe addressed by a method comprising: determining that a nitrogen oxide(NOx) sensor is degraded based on outputs received from the sensor via aCAN bus during a self-diagnostic (SD) test performed after a firstcompleted SD test after a key-off event, only if the outputs aregenerated under conditions where a temperature at the sensor is lessthan a threshold, a NOx concentration is less than a threshold, and anoxygen concentration is within a threshold range.

In another representation, the issues described above may be addressedby a method comprising: excluding a first completed self-diagnostic (SD)test result of an NOx sensor after an engine key-off event, excludingtest results from a completed SD test if one or more of an exhaust gastemperature is greater than a threshold, an oxygen concentration of theexhaust gas is outside a threshold range, and a NOx concentration of theexhaust gas is higher than a threshold, otherwise not excluding testresults from a completed SD test, and determining that the sensor isdegraded only if the non-excluded test results are different from areference value by more than a threshold.

In this way, by excluding SD test results from SD tests where theexhaust gas temperature is greater than a threshold, and/or the NOxconcentration is greater than a threshold, the variance in the SD testresults may be reduced. Said another way, the accuracy of the SD testresults may be improved. As such, the sensitivity for distinguishing adegraded NOx sensor from a NOx sensor that is not degraded may beincreased. Therefore, the efficiency of a NOx emission control system inan exhaust system may be increased.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a schematic diagram of an engine including an exhaustsystem with an exhaust gas treatment system.

FIG. 1B illustrates an exhaust system for receiving engine exhaust gas.

FIG. 2 shows a schematic diagram of an example NOx sensor.

FIG. 3 shows a flow chart of an example method for determining if a NOxsensor is degraded.

FIG. 4 shows a flow chart of an example method for performing aself-diagnostic (SD) test on a NOx sensor.

FIG. 5 shows a flow chart of an example method for excluding outputsfrom a NOx sensor during a SD test to detect a gain skewed-low type ofsensor degradation.

FIG. 6 shows a flow chart of an example method for excluding outputsfrom a NOx sensor during a SD test to detect a gain skewed-high type ofsensor degradation.

DETAILED DESCRIPTION

The following description relates to systems and methods for performinga NOx sensor self-diagnostic (SD) test and determining if a NOx sensoris degraded based on results from the SD test. The exhaust systems ofdiesel engines, such as the engine system shown in FIG. 1, and exhaustsystem shown in FIG. 1B, may comprise a selective catalytic reduction(SCR) catalyst for reducing NOx emissions. The efficiency of the SCRcatalyst may be monitored by one or more NOx sensors positioned upstreamand/or downstream of the SCR catalyst. An example NOx sensor is shownbelow with reference to FIG. 2.

However, the NOx sensor may become degraded (e.g., gain-skewed) overuse, and thus NOx sensors may run self-diagnostic (SD) tests to monitorthe accuracy of their outputs. An example method for running a SD testis shown in FIG. 4. Outputs from the SD test may vary depending on theconcentration of ammonia in the exhaust gas, as ammonia may beregistered as NOx by the NOx sensors. Ammonia concentration in theexhaust gas may increase with increases in an amount of urea and/orexhaust gas temperatures. As such, outputs from the SD test may becomeincreasingly divergent with increasing exhaust gas temperature and/orurea concentration. Therefore, outputs from a NOx sensor that is notdegraded, and outputs from a NOx sensor that is degraded may overlap,and thus, a NOx sensor that is degraded may not be identified.

FIGS. 5-6, show example methods for excluding outputs from SD testresults where the exhaust gas temperature is above a threshold, theoxygen concentration is above a threshold, etc. In this way, thevariance, or spread of results from the SD tests may be reduced. Afterexcluding SD test results based on one or more of exhaust gastemperature, NOx concentration, oxygen concentration, and an order inwhich the SD tests were run, a method such as the example method shownin FIG. 3, may be executed to determine if the NOx sensor is degraded.In this way, a NOx that is degraded may be identified even if exhausttemperatures and/or urea levels in the exhaust system fluctuate.

Referring now to FIG. 1A, a schematic diagram showing one cylinder of amulti-cylinder engine 10, which may be included in a propulsion systemof an automobile, is illustrated. The engine 10 may be controlled atleast partially by a control system including a controller 8 and byinput from a vehicle operator 72 via an input device 70. In thisexample, the input device 70 includes an accelerator pedal and a pedalposition sensor 74 for generating a proportional pedal position signalPP. A combustion chamber (i.e., cylinder) 30 of the engine 10 mayinclude combustion chamber walls 32 with a piston 36 positioned therein.The piston 36 may be coupled to a crankshaft 40 so that reciprocatingmotion of the piston is translated into rotational motion of thecrankshaft. The crankshaft 40 may be coupled to at least one drive wheelof a vehicle via an intermediate transmission system. Further, a startermotor may be coupled to the crankshaft 40 via a flywheel to enable astarting operation of the engine 10.

The combustion chamber 30 may receive intake air from an intake manifold44 via an intake passage 42 and may exhaust combustion gases via anexhaust passage 48. The intake manifold 44 and the exhaust passage 48can selectively communicate with the combustion chamber 30 viarespective intake valve 52 and exhaust valve 54. In some embodiments,the combustion chamber 30 may include two or more intake valves and/ortwo or more exhaust valves.

In the example depicted in FIG. 1A, the intake valve 52 and exhaustvalve 54 may be controlled by cam actuation via respective cam actuationsystems 51 and 53. The cam actuation systems 51 and 53 may each includeone or more cams and may utilize one or more of cam profile switching(CPS), variable cam timing (VCT), variable valve timing (VVT), and/orvariable valve lift (VVL) systems that may be operated by the controller8 to vary valve operation. The position of the intake valve 52 and theexhaust valve 54 may be determined by position sensors 55 and 57,respectively. In alternative embodiments, the intake valve 52 and/orexhaust valve 54 may be controlled by electric valve actuation. Forexample, the cylinder 30 may alternatively include an intake valvecontrolled via electric valve actuation and an exhaust valve controlledvia cam actuation including CPS and/or VCT systems.

In some embodiments, each cylinder of the engine 10 may be configuredwith one or more fuel injectors for providing fuel thereto. As anon-limiting example, the cylinder 30 is shown including one fuelinjector 66. The fuel injector 66 is shown coupled directly to thecylinder 30 for injecting fuel directly therein in proportion to thepulse width of signal FPW received from the controller 8 via anelectronic driver 68. In this manner, the fuel injector 66 provides whatis known as direct injection (hereafter also referred to as “DI”) offuel into the combustion cylinder 30.

It will be appreciated that in an alternate embodiment, the injector 66may be a port injector providing fuel into the intake port upstream ofthe cylinder 30. It will also be appreciated that the cylinder 30 mayreceive fuel from a plurality of injectors, such as a plurality of portinjectors, a plurality of direct injectors, or a combination thereof.

In one example, the engine 10 is a diesel engine that combusts air anddiesel fuel through compression ignition. In other non-limitingembodiments, the engine 10 may combust a different fuel includinggasoline, biodiesel, or an alcohol containing fuel blend (e.g., gasolineand ethanol or gasoline and methanol) through compression ignitionand/or spark ignition.

The intake passage 42 may include a throttle 62 having a throttle plate64. In this particular example, the position of the throttle plate 64may be varied by the controller 8 via a signal provided to an electricmotor or actuator included with the throttle 62, a configuration that iscommonly referred to as electronic throttle control (ETC). In thismanner, the throttle 62 may be operated to vary the intake air providedto the combustion chamber 30 among other engine cylinders. The positionof the throttle plate 64 may be provided to the controller 8 by throttleposition signal TP. The intake passage 42 may include a mass air flowsensor 50 and a manifold air pressure sensor 56 for providing respectivesignals, MAF and MAP, to the controller 8.

Further, in the disclosed embodiments, an exhaust gas recirculation(EGR) system may route a desired portion of exhaust gas from the exhaustpassage 48 to the intake passage 42 via an EGR passage 47. The amount ofEGR provided to the intake manifold 44 may be varied by a controller 8via an EGR valve 49. By introducing exhaust gas to the engine 10, theamount of available oxygen for combustion is decreased, thereby reducingcombustion flame temperatures and reducing the formation of NO_(x) forexample. As depicted, the EGR system further includes an EGR sensor 46which may be arranged within the EGR passage 47 and may provide anindication of one or more of pressure, temperature, and concentration ofthe exhaust gas. Under some conditions, the EGR system may be used toregulate the temperature of the air and fuel mixture within thecombustion chamber, thus providing a method of controlling the timing ofignition during some combustion modes. Further, during some conditions,a portion of combustion gases may be retained or trapped in thecombustion chamber by controlling exhaust valve timing, such as bycontrolling a variable valve timing mechanism.

An exhaust system 2 includes an exhaust gas sensor 26 coupled to theexhaust passage 48 upstream of an exhaust gas treatment system 80. Thesensor 26 may be any suitable sensor for providing an indication ofexhaust gas air/fuel ratio such as a linear oxygen sensor or UEGO(universal or wide-range exhaust gas oxygen), a two-state oxygen sensoror EGO, a HEGO (heated EGO), a NO_(x), HC, or CO sensor. The exhaust gastreatment system 80 is shown arranged along the exhaust passage 48downstream of the exhaust gas sensor 26.

In the example shown in FIG. 1A, the exhaust gas treatment system 80 isa urea based selective catalytic reduction (SCR) system. The SCR systemincludes at least a reduction catalyst (herein, SCR catalyst 82), areductant storage tank (herein, urea storage reservoir 84), and areductant injector (herein, urea injector 96), for example. In otherembodiments, the exhaust gas treatment system 80 may additionally oralternatively include other components, such as a particulate filter,lean NO_(x) trap, three way catalyst, various other emission controldevices, or combinations thereof. For example, urea injector 96 may bepositioned upstream of reduction catalyst 82 and downstream of anoxidation catalyst. In the depicted example, the urea injector 96provides urea from the urea storage reservoir 84. However, variousalternative approaches may be used, such as solid urea pellets thatgenerate an ammonia vapor, which is then injected or metered to the SCRcatalyst 82. In still another example, a lean NO_(x) trap may bepositioned upstream of the SCR catalyst 82 to generate NH₃ for the SCRcatalyst 82, depending on the degree or richness of the air-fuel ratiofed to the lean NO_(x) trap.

The exhaust gas treatment system 80 further includes a tailpipe exhaustgas sensor 97 positioned downstream of the SCR catalyst 82. In thedepicted embodiment, tailpipe exhaust gas sensor 97 may be a NO_(x)sensor, for example, for measuring an amount of post-SCR NO_(x)releasedvia the tailpipe of exhaust passage 48. Exhaust gas treatment system 80may further include a feedgas exhaust gas sensor 90 positioned upstreamof the SCR catalyst 82 and downstream of urea injector 96. In thedepicted embodiment, the feedgas exhaust gas sensor 90 may also be aNO_(x) sensor, for example, for measuring an amount of pre-SCR NO_(x)received in the exhaust passage for treatment at the SCR catalyst.

In some examples, an efficiency of the SCR system may be determinedbased on the output of one or more of tailpipe exhaust gas sensor 97 andfeedgas exhaust gas sensor 90. For example, the SCR system efficiencymay be determined by comparing NO_(x) levels upstream of the SCRcatalyst (via sensor 90) with NO_(x) levels downstream of the SCRcatalyst (via sensor 97). The efficiency may be further based on theexhaust gas sensor 26 (when the sensor 26 measures NO_(x), for example)positioned upstream of the SCR system. In other examples, exhaust gassensors 97, 90, and 26 may be any suitable sensor for determining anexhaust gas constituent concentration, such as a UEGO, EGO, HEGO, HC, COsensor, etc.

The controller 8 is shown in FIG. 1A as a microcomputer, including amicroprocessor unit 16, input/output ports 4, an electronic storagemedium for executable programs and calibration values shown as a readonly memory chip 14 in this particular example, random access memory 18,keep alive memory 20, and a data bus. The controller 8 may be incommunication with and, therefore, receive various signals from sensorscoupled to the engine 10, in addition to those signals previouslydiscussed, including measurement of inducted mass air flow (MAF) fromthe mass air flow sensor 50; engine coolant temperature (ECT) from atemperature sensor 58 coupled to a cooling sleeve 61; a profile ignitionpickup signal (PIP) from a Hall effect sensor 59 (or other type) coupledto the crankshaft 40; throttle position (TP) from a throttle positionsensor; absolute manifold pressure signal, MAP, from the sensor 56; andexhaust constituent concentration from the exhaust gas sensors 26, 90,and 97. An engine speed signal, RPM, may be generated by the controller8 from signal PIP.

The storage medium read-only memory 14 can be programmed withnon-transitory, computer readable data representing instructionsexecutable by the processor 16 for performing the methods describedbelow as well as other variants that are anticipated but notspecifically listed. Example methods are described herein with referenceto FIGS. 3-4.

As described above, FIG. 1A shows only one cylinder of a multi-cylinderengine, and each cylinder may similarly include its own set ofintake/exhaust valves, fuel injector, spark plug, etc.

FIG. 1B illustrates a schematic view of an example exhaust system 102 ofa vehicle system for transporting exhaust gases produced by internalcombustion engine 110. Exhaust system 102 may be exhaust system 2 ofFIG. 1A, for example. As one non-limiting example, engine 110 includes adiesel engine that produces a mechanical output by combusting a mixtureof air and diesel fuel. Alternatively, engine 110 may include othertypes of engines such as gasoline burning engines, among others.

Exhaust system 102 may include one or more of the following: an exhaustmanifold 120 for receiving exhaust gases produced by one or morecylinders of engine 110, an oxidation catalyst 124, mixing region 130,selective catalytic reductant (SCR) catalyst 140, emission controldevice 142, and noise suppression device 150. Additionally, exhaustsystem 102 may include a plurality of exhaust pipes or passages forfluidically coupling the various exhaust system components of exhaustsystem 102. It is important to note that one or more of the oxidationcatalyst 124, mixing region 130, SRC catalyst 140, emission controldevice 142, and noise suppression device 150 may be arranged in anyorder or combination in the exhaust system 102.

However, as shown the example of FIG. 1B, exhaust manifold 120 may befluidically coupled to oxidation catalyst 124 by one or more of firstexhaust passage 162 and second exhaust passage 164. As, such oxidationcatalyst 124 may be downstream of exhaust manifold 120, with noadditional components separating the oxidation catalyst 124 from theexhaust manifold 120 other than one or more of the exhaust passages 162and 164. In the description herein, the flow of gasses and/or fluids inthe exhaust system 102 may be in a direction away from exhaust manifold120, towards surrounding environment 195, through the exhaust system102, and out of the exhaust system 102 through passage 168. Thus, in theexample shown in FIG. 1B, the flow of gasses and/or fluids in theexhaust system 102 may generally be from left to right as indicated byflow arrow 180. Therefore, in the description herein, downstream may beused to refer to the relative positioning of components in the exhaustsystem 102 with respect to the flow direction in the exhaust system 102.As such, if a first component is described as downstream relative to asecond component in the exhaust system 102, then gasses and/or fluidsflowing in the exhaust system 102 may flow through the second componentbefore flowing through first component.

Returning now to FIG. 1B, one or more of the first exhaust passage 162and second exhaust passage 164 may provide fluidic communication betweenthe exhaust manifold 120 and the oxidation catalyst. In some example theoxidation catalyst 120 may be a diesel oxidation catalyst (DOC). The DOCmay be an exhaust flow through device that includes a honey-comb formedsubstrate having a large surface area coated with a catalyst layer. Thecatalyst layer may include precious metals including, but not limitedto, platinum and palladium. As the exhaust flows over the catalystlayer, carbon monoxide, gaseous hydrocarbons and liquid hydrocarbonparticles may be oxidized to reduce emissions.

The mixing region 130 may be arranged immediately downstream ofoxidation catalyst 124 for receiving a liquid reductant, with noadditional components separating the mixing region 130 from theoxidation catalyst 124. The mixing region 130 may include a first mixingregion 132, and a second mixing region 134, where the second mixingregion 134 is downstream of the first mixing region 132. The firstmixing region 132 may include an injector 136, for injecting a liquidinto the mixing region 130. In some examples, the liquid injected by theinjector 136, may be a liquid reductant such as ammonia or urea. Theliquid reductant may be supplied by to the injector 136 from a storagetank (not shown). Further, the injector 136 may comprise anelectronically actuated valve. In such examples, the injector 136 may bein electrical and/or electronic communication with controller 112, forreceiving signals from the controller 112. As such, controller 112, maysend electronic signals to the injector 136, for adjusting a position ofthe valve of the injector. In response to signals received from thecontroller 112, an actuator of the injector 136 may adjust the positionof the valve of the injector 136, to adjust an amount of liquidreductant being injected to the mixing region 130.

Additionally, the first mixing region 132 may comprise an upstream firstnitrogen oxide (NOx) sensor 190, and an upstream first temperaturesensor 191. The positioning of the first NOx sensor 190 and the firsttemperature sensor 191 in the exhaust system 102 may be such that theNOx sensor 190 and temperature sensor 191 are superposed. Said anotherway, the first NOx sensor 190 and the first temperature sensor 191 maybe approximately aligned with, and may coincide with one another in theexhaust system 102. Said another way, the NOx sensor 190 and temperaturesensor 191 may be radially arranged along a cross section of the flowpath of exhaust gasses in the exhaust system 102. In some examples, theNOx sensor 190 and the first temperature sensor 191, may be arrangedperpendicular to the flow of gasses and/or fluid in the exhaust system102, and in such examples, may further be positioned in the exhaustsystem 102 so that they are parallel to one another. In other examples,the temperature sensor 191 may be positioned directly adjacent to theNOx sensor 190, so that the temperature sensor 191 and NOx sensor 190are in face-sharing contact with one another. In this way, gasses and/orfluids flowing through the exhaust system 102, and more specificallythrough first mixing region 132, may flow past the first NOx sensor 190and the first temperature sensor 191 at approximately the same time. Assuch, the first temperature sensor 191 may be positioned within thefirst mixing region 132 for measuring a temperature of gasses and/orfluids flowing past and/or being sampled at the first NOx sensor 190.

However, in other examples, the temperature sensor 191 may not bealigned with the NOx sensor 190. Thus, the temperature sensor 191 may bepositioned adjacent to the NOx sensor 190, and/or may be spaced awayfrom the NOx sensor 190.

The temperature sensor 191 may be electrically coupled to the controller112, where outputs of the temperature sensor 191 corresponding to atemperature of gasses and/or fluids flowing past the NOx sensor 190 maybe sent to the controller 112. In some examples, the first NOx sensor190 and first temperature sensor 191 may be positioned downstream of theinjector 136 as shown in FIG. 1B. However, in other examples, the firstNOx sensor 190 and first temperature sensor 191, may be positionedsubstantially in line with injector 136. In still further examples, theNOx sensor 190 and temperature sensor 191 may be positioned upstream ofthe injector 136.

The second mixing region 134 of mixing region 130 may be configured toaccommodate a change in cross-sectional area or flow area between thefirst mixing region 132 and the catalyst 140. Specifically, thecross-sectional flow area created by the second mixing region 134 mayincrease in a downstream direction. As such, catalyst 140 may bepositioned downstream of mixing region 130. Therefore, the first NOxsensor 190, and the first temperature sensor 191 are positioned upstreamof the SCR catalyst 140. In some examples, no additional components mayseparate the second mixing region 134 from the SCR catalyst 140. In someexamples, a mixing device 138 may be included in the exhaust system 120and may be arranged downstream of injector 136. Mixing device 138 may beconfigured to receive engine exhaust gas and/or injected fluid reductantfrom injector 136 from upstream of the mixing device 138 and direct theengine exhaust gas and/or fluid reductant downstream of the mixingdevice 138 towards the SCR catalyst 140. As shown in FIG. 1B, mixingdevice 138 may comprise a circular disc of fin sections. Each finsection may have a straight edge and a curved edge. In some examples,the mixing device 138 may be positioned in the first mixing region 132downstream of the injector 136, temperature sensor 191, and NOx sensor190. In other examples, the mixing device 138 may be positioned in thesecond mixing region 134. The mixing device 138, may increase thecommingling and therefore uniformity of the exhaust gas and/or fluidreductant mixture in the second mixing region 134 before the mixturereaches the SCR catalyst 140.

The SCR device 140 is configured to convert NOx gasses into water andnitrogen as inert byproducts of combustion using the fluid reductant,e.g., ammonia (NH3) or urea, injected by the injector 136 and an activecatalyst. The catalyst, often referred to as a DeNOx catalyst, may beconstructed of titanium dioxide containing the oxides of transitionmetals such as, for example, vanadium, molybdenum, and tungsten to actas catalytically active components. The SCR device 124 may be configuredas a ceramic brick or a ceramic honeycomb structure, a plate structure,or any other suitable design. Note that catalyst 140 can include anysuitable catalyst for reducing NOx or other products of combustionresulting from the combustion of fuel by engine 110.

The emission control device 142 may be positioned downstream of the SCRcatalyst 140. In some examples, the emission control device 142 may be adiesel particulate filter (DPF). The DPF may operate actively orpassively, and the filtering medium can be of various types of materialand geometric construction. One example construction includes awall-flow ceramic monolith comprising alternating channels that areplugged at opposite ends, thus forcing the exhaust flow through thecommon wall of the adjacent channels whereupon the particulate matter isdeposited.

While in the example shown in FIG. 1B the SCR catalyst 140 is upstreamof the emission control device 142, in other examples, the emissioncontrol device 142 may be positioned upstream of the SCR catalyst 140.However, in all such examples, the first NOx sensor 190 and the firsttemperature sensor 191 are positioned upstream of the SCR catalyst 140and the emission control device 142.

Still in further examples, it is possible that the emission controldevice 142 and the SCR catalyst 140 may be combined on one substrate(e.g., a wall-flow ceramic DPF element coated with NOx storage agentsand platinum group metals).

After passing through the emission control device 142, exhaust gassesand/or fluids may continue through an after-treatment region 144. Theafter-treatment region 144 may be configured to accommodate a change incross-sectional area or flow area between the emission control device142 and third exhaust passage 166. Specifically, the cross-sectionalflow area created by the after-treatment mixing region 144 may decreasein a downstream direction. Thus, the after-treatment mixing region 144may fluidically couple the emission control device 142 to the thirdexhaust passage 166. However, in other examples, the exhaust system 102may not include after-treatment mixing region 144, and as such, theemission control device 142 may be directly and/or physically coupled tothe third exhaust passage 166, with no additional components separatingthe emission control device 142 from the third exhaust passage 166.

A downstream second temperature sensor 193, and a downstream second NOxsensor 192 may be positioned in the third exhaust passage 166. However,in other examples, the second temperature sensor 193, and the second NOxsensor 192 may be positioned in the after-treatment mixing region 144.In all examples, the second temperature sensor 193, and the second NOxsensor 192 are positioned downstream of the SCR catalyst 140 and theemission control device 142. The second temperature sensor 193 andsecond NOx sensor 192 may be positioned within the exhaust system 102 ina similar manner to that of first temperature sensor 191 and first NOxsensor 190. As such, the second NOx sensor 192 and second temperaturesensor 193 may superposed in the exhaust system 102. Said another way,the second NOx sensor 192 and the second temperature sensor 193 may beapproximately aligned with, and may coincide with one another in theexhaust system 102.

Thus, in some examples, the NOx sensor 192 and the temperature sensor193, may be arranged perpendicular to the flow of gasses and/or fluid inthe exhaust system 102, and in such examples, may further be positionedin the exhaust system 102 so that they are parallel to one another. Inthis way, gasses and/or fluids flowing through the exhaust system 102,and more specifically flowing downstream of the SCR catalyst 140 andemission control device 142, may flow past the second NOx sensor 192 andthe second temperature sensor 193 at approximately the same time. Assuch, the second temperature sensor 193 may be positioned within theexhaust system 102 for measuring a temperature of gasses and/or fluidsflowing past and/or being sampled at the second NOx sensor 192. Thetemperature sensor 193 may be electrically coupled to the controller112, where outputs of the temperature sensor 193 corresponding to atemperature of gasses and/or fluids flowing past and/or being sampled atthe NOx sensor 192 may be sent to the controller 112.

However, in other examples, the temperature sensor 193 may not bealigned with the NOx sensor 192. Thus, the temperature sensor 193 may bepositioned adjacent to the NOx sensor 191, and/or may be spaced awayfrom the NOx sensor 192.

Noise suppression device 150 may be arranged downstream of catalyst 140and emission control device 142. Noise suppression device 150 may beconfigured to attenuate the intensity of sound waves traveling away fromexhaust manifold 120, towards surrounding environment 195. Third exhaustpassage 166 may provide fluidic communication between after-treatmentregion 144 and noise suppression device 150. Thus, exhaust gases may bepermitted to flow from the after-treatment region 144, through thirdexhaust passage 166, to noise suppression device 150. After passingthrough noise suppression device 150, exhaust gasses may flow throughfourth exhaust passage 168 en route to the surrounding environment 195.

Returning to the first NOx sensor 190 and second NOx sensor 192, both ofthe sensors 190 and 192 may be constructed and function similarly. Theymay both be configured to measure and/or estimate a concentration of NOxand/or O₂ in an exhaust gas mixture flowing through the exhaust system102. The structure and functioning of the sensors 190 and 192 may bedescribed in greater detail below with reference to FIG. 2.

The first NOx sensor 190 may be electrically coupled to a first NOxsensor module 194, and the second NOX sensor 192 may be electricallycoupled to a second NOX sensor module 198. The vehicle system 100 mayalso include Controller Area Network (CAN) bus 152 in communication withthe exhaust system 102 and controller 112. The CAN bus may exchangeinformation using a scheduled periodic rate. As such, the CAN bus 152provides electronic communication between the controller 112, and thefirst NOx sensor module 194 and the second NOx sensor module 198.Measurements from the two NOx sensors 190 and 192 (e.g. NOxconcentration, O₂ concentration, etc.) are first relayed to the NOXsensor modules 194 and 198, respectively. The NOx sensor modules 194 and198 may then convert signals received from the NOx sensors 190 and 192,respectively, into a CAN data stream, which may then be transmitted tothe controller 112 via the CAN bus 152. The NOx sensor modules 194 and198 may also include computer readable instructions stored innon-transitory memory for determining whether a self-diagnostic (SD)test is complete.

During engine operation, each of the NOx sensors 190 and 192 may operatein a first mode, as described in greater detail below with reference toFIG. 2, to measure and/or estimate NOx levels in the exhaust system 102.The upstream first NOx sensor measures NOx levels emitted by the engine110, while the downstream second NOx sensor measures NOx levelsremaining in the exhaust system 102 after treatment by the SCR catalyst140. By comparing the outputs of the two NOx sensors 190 and 192 duringengine operation, the overall NOx removal efficiency of the exhaustsystem 102 may be estimated.

However, the NOx sensors 190 and 192 may become degraded (e.g.,gain-skewed, cracked, contaminated, etc.), and as a result the accuracyof their outputs used to estimate and/or measure NOx levels in theexhaust system 102 may become reduced. In order to detect, and diagnoseNOx sensor degradation, the accuracy of a sensor's outputs may bemonitored after an engine key-off event as described in greater detailbelow with reference to FIG. 3. Thus, in a second mode of operation,after an engine key-off event, the NOx sensors 190 and 192 may run oneor more self-diagnostic (SD) tests to determine if one or more of theNOx sensors 190 and 192 are degraded. A key-off event may be determinedby the controller 112 based on signals received from an input device170. The input device 170 may include a button, switch, knob, ignition,touch screen display, etc., where the position and/or digital state ofthe input device 170 may be adjusted to turn the engine 110 on or off.The input device 170 may therefore in some examples by an ignition for avehicle system with a keyed engine-on, engine-off functionality. Inother embodiments, in the case of a keyless vehicle system, thestart/stop and/or on/off functionality of the vehicle system may becontrolled by a button, switch, knob, touch screen, etc. Thus, a vehicleoperator 172, may adjust the input device 170 to a first position and/ordigital state during a key-on event to turn on the engine 110. During akey-off event the vehicle operator 172, may adjust the position of theinput device to a second position and/or digital state during a key-offevent to turn off the engine 110. Said another way, a key-off event maybe used to refer to conditions when the engine 110 is shutdown to restand the vehicle is off (e.g., during an engine key-off and/or vehiclekey-off event, or engine stop event in a keyless system with astop/start button). Thus, the key-off event may comprise terminating acombustion cycle in the engine 110 based on input from the vehicleoperator 172 via an input device. In some examples, the vehicle system100 may additionally include a position sensor 174, for measuring aposition of the input device 170. The position of the input device 170may be transmitted to the controller 112, and may thus be used by thecontroller 112 to determine the occurrence of a key-off and/or key-onevent.

Power to the NOx sensors 190 and 192 after an engine key-off event mayin some examples be provided by a glow plug control module 156. However,in other examples, power to the NOx sensor 190 and 192 may be providedby a battery 184. Thus, in a second mode of operation, after an enginekey-off event, the NOx sensors 190 and 192 may run one or moreself-diagnostic (SD) tests to determine if one or more of the NOx sensor190 and 192 are degraded. During this second mode of operation, wherethe NOx sensors 190 and 192 run one or more SD tests, the glow plugcontrol module 156 and/or battery 184, may provide power to the upstreamfirst NOx sensor 190 and the downstream second NOx sensor 192. The glowplug control module 156 and battery 184 may be in electricalcommunication with the controller 112, for receiving digital signalstherefrom. In some examples, the glow plug control module 156, may be incommunication with the controller 112 via the CAN bus 152. Thecontroller 112 may in some examples initiate an engine after-run routinewhere a required amount of after-run extension time may be transmittedto the glow plug control module (GPCM) 156 via signal 143, while theengine 110 is running.

As described in greater detail below with reference to FIG. 3, thecontroller 112 may comprise computer readable instructions stored innon-transitory memory for initiating an engine after-run routine after akey-off event. Controller 112 may determine and transmit a desiredduration (e.g. amount of time, number of SD test cycles, etc.) for theafter-run routine to the glow plug control module (GPCM) 156. Thus, thedesired duration may be an amount of time the after a key-off event thatthe GPCM 156 may continue to supply power to one or more components ofthe vehicle system 100, such as one or more of the NOx sensor 190 and192. In some examples, the controller 112 may transmit the desiredduration via signal 143 to the GPCM 156, while the engine 110 isrunning, which in some examples may be before a key-off event. However,in other examples the desired duration may be transmitted to the GPCMafter a key-off event. The engine after-run routine is defined when theengine is turned off after a key-off event, but power is still suppliedto the exhaust system 102 and CAN bus 152 with the GPCM 156. Further,the controller 112 may send signals to one or more of the NOx sensors190 and 192 after a key-off event to initiate and/or run one or more SDtests.

Controller 112 is shown in FIG. 1B as a microcomputer including:microprocessor unit 106, input/output ports 104, read-only memory 105,random access memory 108, keep alive memory 109, and a conventional databus. Controller 112 is shown receiving various signals from sensorscoupled to exhaust system 102, in addition to those signals previouslydiscussed, including: exhaust gas temperature from temperature sensors191 and 193 which may be coupled to first mixing region 132 and thirdexhaust passage 166 respectively; a position sensor 174 coupled to aninput device 170 for sensing input device position adjusted by a vehicleoperator 172; and NOx levels from NOx sensors 190 and 192 positionedupstream and downstream of the SCR catalyst 140, respectively.

Note that with regards to vehicle applications, exhaust system 102 maybe arranged on the underside of the vehicle chassis. Additionally, itshould be appreciated that the exhaust system 102 may include one ormore bends or curves to accommodate a particular vehicle arrangement.Further still, it should be appreciated that in some embodiments,exhaust system 102 may include additional components not illustrated inFIG. 1B and/or may omit components described herein.

FIG. 2 shows a schematic view of an example embodiment of a NOx sensor200 configured to measure a concentration of NOx gases in an emissionsstream. Sensor 200 may operate as the NOx sensor 190 or 192 of FIG. 1B,for example. Sensor 200 comprises a plurality of layers of one or moreceramic materials arranged in a stacked configuration. In the embodimentof FIG. 2, six ceramic layers are depicted as layers 201, 202, 203, 204,205, and 206. These layers include one or more layers of a solidelectrolyte capable of conducting ionic oxygen. Examples of suitablesolid electrolytes include, but are not limited to, zirconiumoxide-based materials. Further, in some embodiments, a heater 232 may bedisposed between the various layers (or otherwise in thermalcommunication with the layers) to increase the ionic conductivity of thelayers. While the depicted NOx sensor is formed from six ceramic layers,it will be appreciated that the NOx sensor may include any othersuitable number of ceramic layers.

Layer 202 includes a material or materials creating a first diffusionpath 210. First diffusion path 210 is configured to introduce exhaustgases from an exhaust passage (e.g., first mixing region 132 shown inFIG. 1B) into a first internal cavity 212 via diffusion. A first pair ofpumping electrodes 214 and 216 are disposed in communication withinternal cavity 212, and may be configured to electrochemically pump aselected exhaust gas constituent from internal cavity 212 through layer201 and out of sensor 200. Generally, the species pumped from internalcavity 212 out of sensor 200 may be a species that may interfere withthe measurement of a desired analyte. For example, molecular oxygen(e.g., 02) can potentially interfere with the measurement of NOx in aNOx sensor, as oxygen is dissociated and pumped at a lower potentialthan NOx. Therefore, first pumping electrodes 214 and 216 may be used toremove molecular oxygen from within internal cavity 212 to decrease theconcentration of oxygen within the sensor relative to a concentration ofNOx within the sensor.

First diffusion path 210 may be configured to allow one or morecomponents of exhaust gases, including but not limited to the analyteand interfering component, to diffuse into internal cavity 212 at a morelimiting rate than the interfering component can be electrochemicallypumped out by first pumping electrodes pair 214 and 216. In this manner,almost all of oxygen may be removed from first internal cavity 212 toreduce interfering effects caused by oxygen. Herein, the first pumpingelectrodes pair 214 and 216 may be referred to as an O2 pumping cell.

The process of electrochemically pumping the oxygen out of firstinternal cavity 212 includes applying an electric potential VIp0 acrossfirst pumping electrode pair 214, 216 that is sufficient to dissociatemolecular oxygen, but not sufficient to dissociate NOx. With theselection of a material having a suitably low rate of oxygen diffusionfor first diffusion path 210, the ionic current Ip0 between firstpumping electrode pair 214, 216 may be limited by the rate at which thegas can diffuse into the chamber, which is proportional to theconcentration of oxygen in the exhaust gas, rather than by the pumpingrate of the O2 pumping cell. This may allow a substantial majority ofoxygen to be pumped from first internal cavity 212 while leaving NOxgases in first internal cavity 212. A voltage V0 across first pumpingelectrode 214 and reference electrode 228 may be monitored to providefeedback control for the application of the electric potential VIp0across first pumping electrode pair 214, 216.

Sensor 200 further includes a second internal cavity 220 separated fromthe first internal cavity by a second diffusion path 218. Seconddiffusion path 218 is configured to allow exhaust gases to diffuse fromfirst internal cavity 212 into second internal cavity 220. A secondpumping electrode 222 optionally may be provided in communication withsecond internal cavity 220. Second pumping electrode 222 may, inconjunction with electrode 216, be set at an appropriate potential VIp1to remove additional residual oxygen from second internal cavity 220.Second pumping electrode 222 and electrode 216 may be referred to hereinas a second pumping electrode pair or a residual O2 monitoring cell.Alternatively, second pumping electrode 222 may be configured tomaintain a substantially constant concentration of oxygen within secondinternal cavity 220. In some embodiments, (VIp0) may be approximatelyequal to (VIp1) while in other embodiments (VIp0) and (VIp1) may bedifferent. While the depicted embodiment utilizes electrode 216 to pumpoxygen from first internal cavity 212 and from second internal cavity220, it will be appreciated that a separate electrode (not shown) may beused in conjunction with electrode 222 to form an alternate pumpingelectrode pair to pump oxygen from second internal cavity 220. A voltageV1 across second pumping electrode 222 and reference electrode 228 maybe monitored to provide feedback control for the application of theelectric potential VIp1 across second pumping electrode pair 222, 216.

First pumping electrode 212 and second pumping electrode 222 may be madeof various suitable materials. In some embodiments, first pumpingelectrode 212 and second pumping electrode may be at least partiallymade of a material that catalyzes the dissociation of molecular oxygento the substantial exclusion of NOx. Examples of such materials include,but are not limited to, electrodes containing platinum and/or gold.

Sensor 200 further includes a measuring electrode 226 and a referenceelectrode 228. Measuring electrode 226 and reference electrode 228 maybe referred to herein as a measuring electrode pair. Reference electrode228 is disposed at least partially within or otherwise exposed to areference duct 230. In one embodiment, reference duct 230 may be open tothe atmosphere and may be referred to as a reference air duct. Inanother embodiment, reference duct 230 may be isolated by a layer 236from the atmosphere such that oxygen pumped from second internal cavity220 may be accumulated within the duct, thus reference duct 230 may bereferred to as an oxygen duct.

Measuring electrode 226 may be set at a sufficient potential relative toreference electrode 228 to pump NOx out of second internal cavity 220.Further, measuring electrode 226 may be at least partially made of amaterial that catalyzes dissociation or reduction of any NOx. Forexample, measuring electrode 226 may be made at least partially fromplatinum and/or rhodium. As NOx is reduced to N2, the oxygen ionsgenerated are electrochemically pumped from second internal cavity 220.The sensor output is based upon the pumping current flowing throughmeasuring electrode 226 and reference electrode 228, which isproportional to the concentration of NOx in second internal cavity 220.Thus, the pair of electrodes 226 and 228 may be referred to herein as aNOx pumping cell.

Sensor 200 further includes a calibration electrode 234. Calibrationelectrode 234 is used to measure the residual oxygen concentration insecond internal cavity 220 according to a Nernst voltage (Vn) withreference to reference electrode 228. Thus, calibration electrode 234and reference electrode 228 may be referred to herein as a calibrationelectrode pair or as a residual O2 monitoring cell. As shown in FIG. 2,calibration electrode 234 is disposed on the same solid electrolytelayer 203 as measuring electrode 226. Typically, calibration electrode234 is disposed spatially adjacent to measuring electrode 226. The term“spatially adjacent” as used herein refers to the calibration electrode234 being in the same volume of space (for example, second internalcavity 220) as measuring electrode 226. Furthermore, placing thecalibration electrode 234 in close proximity to measuring electrode 226may reduce the magnitude of any differences in oxygen concentration atthe measuring electrode and at the calibration electrode due to anoxygen concentration gradient between the two electrodes. This may allowresidual oxygen concentrations to be measured more accurately.Alternatively, calibration electrode 234 and measuring electrode 226 maybe disposed on different solid electrolyte layers. For example,calibration electrode 234 may be disposed on solid electrolyte layer 201instead of layer 203.

It will be appreciated that the depicted calibration electrode locationsand configurations are merely exemplary, and that calibration electrode234 may have various suitable locations and configurations that allows ameasurement of residual oxygen to be obtained. Further, while thedepicted embodiment utilizes electrode 228 as a reference electrode ofthe calibration electrode pair, it will be appreciated that a separateelectrode (not shown) may be used in conjunction with calibrationelectrode 234 to form an alternative calibration electrode pairconfiguration.

It should be appreciated that the NOx sensors described herein aremerely example embodiments of NOx sensors, and that other embodiments ofNOx sensors may have additional and/or alternative features and/ordesigns. For example, in some embodiments, a NOx sensor may include onlyone diffusion path and one internal cavity, thereby placing the firstpumping electrode and measuring electrode in the same internal cavity.In such an embodiment, a calibration electrode may be disposed adjacentto the measuring electrode so that the residual oxygen concentration ofan exhaust gas at or near the measuring electrode can be determined witha minimized impact from any oxygen concentration gradient.

In a first mode of operation, where the NOx concentration may beestimated and/or measured, the electric potential applied across thesecond pumping electrode pair, may be greater than the electricpotential applied across the first pumping electrode pair. In someexamples, VIp0 may be approximately 425 mV and VIp1 may be approximately450 mV. VIp1 may be controlled to a constant voltage to regulate theconcentration of O₂ in the second internal cavity 220. Thus, VIp1 isregulated in the first mode of operation to maintain the oxygenconcentration in the second internal cavity 220 to a lower first level.In some examples the first level of O₂ may be approximately 10⁻³ ppm.Measuring electrode 226 may be set at a sufficient potential relative toreference electrode 228 to pump NOx out of second internal cavity 220.As NOx is reduced to N2, the oxygen ions generated are electrochemicallypumped from second internal cavity 220. The sensor output is based uponthe pumping current flowing through measuring electrode 226 andreference electrode 228, which in the first mode of operation may beproportional to the concentration of NOx in second internal cavity 220.

During a second mode of operation, VIp1 may be controlled to maintainthe oxygen concentration in the second internal cavity to a highersecond level. In some examples, the second level of O₂ may beapproximately 1000 ppm. Measuring electrode 226 may be set at asufficient potential relative to reference electrode 228 to pump O₂ outof second internal cavity 220. The sensor output is based upon thepumping current flowing through measuring electrode 226 and referenceelectrode 228, which in the second mode of operation may be proportionalto the concentration of O₂ in second internal cavity 220.

As explained below with reference to FIG. 4, after an engine key-offevent, the NOx sensor may run one or more SD tests. Each test maycomprise operating the NOx sensor in both the first mode and second modeof operation. Specifically, during a SD test, the NOx sensor may firstrun in the second mode of operation and maintain the O₂ concentration inthe second cavity 220 to a second level (e.g., 1000 ppm). Then, the NOxsensor may run in the first mode of operation, to obtain measurementsand/or estimates for the O₂ and NO_(x) levels. Thus, a completed SD testmay be a test in which the NOx sensor has run both in the first mode andsecond mode of operation. As such, during each completed SD test, NOxand O₂ levels may be measured and/or estimated.

In this way, a method may comprise diffusing a portion of exhaust gassesflowing in an exhaust system into a first cavity of a NOx sensor. Themethod may then comprise applying a first electric potential across afirst pumping cell, and pumping oxygen molecules from the first cavityout of the NOx sensor. A portion of the exhaust gasses admitted to thefirst cavity and not pumped out of the first cavity may then diffuseinto a second cavity of the NOx sensor. Additionally, the method maycomprise applying a second electric potential across a second pumpingcell, and pumping oxygen molecules from the second cavity out of the NOxsensor. In a first mode of operation the concentration of oxygen in thesecond cavity may be adjusted to a lower first amount. In some examples,the lower first amount may be approximately 10⁻³ ppm. In a second modeof operation, the concentration of oxygen in the second cavity may beadjusted to a higher second amount. In some examples, the higher secondamount may be approximately 1000 ppm.

The concentration of oxygen in the second cavity may be adjusted byadjusting one or more of the first and second electric potentialsapplied across the first and second pumping cells, respectively. Thus,in the first mode, the first electric potential applied across the firstpumping cell may be greater in the first mode than that applied duringthe second mode. In other examples, the second electric potentialapplied across the second pumping cell may be greater in the first modethan that applied during the second mode. In still further examples, thesecond electric potential applied across both the first and secondpumping cells may be greater in the first mode than in the second mode.An oxygen concentration in the exhaust gasses may be estimated based onone or more of a first pumping current resulting from oxygen moleculesbeing pumped out of the first cavity. In some examples, the oxygenconcentration may additionally, or alternatively be estimated based on asecond pumping current resulting from oxygen molecules being pumped outof the second cavity.

The method may further comprise, applying a third electric potentialacross a measuring electrode pair. In the first mode the third electricpotential may be sufficiently high to dissociate NOx molecules. Thus, insome examples the third electric potential may be 450 mV. Oncedissociated, NOx molecules may be pumped out of the second cavity, and aconcentration of the NOx may be estimated a third pumping currentresulting from oxygen exiting the second cavity. However, in the secondmode, the third electric potential may be sufficiently high todissociate oxygen molecules, and as such, in some examples, in thesecond mode of operation, the third electric potential may be less than450 mV. During the second mode of operation, oxygen molecules in thesecond cavity may be dissociated upon the application of the thirdelectric potential. In some examples, an estimation of the oxygenconcentration of the exhaust gasses in the second cavity may beestimated based on the resulting pumping.

Turning now to FIG. 3, it shows an example method 300 for determining ifa NOx sensor (e.g. NOx sensor 190, 192 shown in FIG. 1B, NOX sensor 200shown in FIG. 2) is degraded based on outputs from the sensor during oneor more self-diagnostic (SD) tests. As explained above with reference toFIGS. 1-2, the NOx sensor may perform one or more SD tests after akey-off event. However, the concentration of different constituents inthe exhaust gas after an engine key-off event may vary. As a result, theoutputs of the NOx sensor may be different depending on the exhaust gasproperties in an exhaust system (e.g., exhaust system 102 shown in FIG.1B) at the time an SD test is performed. Outputs from a NOx sensor thatis not degraded may vary as much or more between two or more SD tests,than outputs from a NOx sensor that is not degraded and a NOx sensorthat is degraded. Therefore, it may be difficult to diagnose and detectNOx sensor that is degraded. The example method 300 shown in FIG. 3shows a method for determining that a NOx sensor is degraded byexcluding SD test results under certain exhaust gas conditions.

Instructions for carrying out method 300 may be executed by a controller(e.g., controller 112 shown in FIG. 1B) based on instructions stored ona memory of the controller and in conjunction with signals received fromvarious sensors of the engine system, such as the sensors describedabove with reference to FIGS. 1A-1B. Specifically, the controller mayexecute method 300 based on outputs from the NOx sensor received from aNOx sensor control module (e.g., NOx sensor modules 194 and 198 shown inFIG. 1B). The controller may employ engine actuators of the enginesystem to adjust engine operation, according to the methods describedbelow.

Method 300 begins at 302, which comprises estimating and/or measuringengine operating conditions. Engine operating conditions may include anexhaust gas temperature, exhaust gas NOx and/or O₂ concentration, enginetemperature, an intake manifold vacuum, a position of an intake valve, aposition of a throttle valve, etc.

After estimating and/or measuring engine operating conditions at 302,method 300 may then continue to 304 and determine if a key-off event hasoccurred. As described above with reference to FIGS. 1A-1B, an enginekey-off event may be where an engine (e.g. engine 110 shown in FIG. 1B)is turned off by a vehicle operator (e.g., vehicle operator 172 shown inFIG. 1B). Thus, the method at 302 may include determining if an inputdevice controlling the operational status of the engine (e.g., ignitionin a keyed vehicle, push-button in a keyless vehicle, etc.), has beenadjusted to a position signaling the engine to turn off. If it isdetermined at 304 that a key-off event has not occurred, then the method300 may proceed to 306, which comprises not performing an SD test. Thus,if a key-off event has not occurred, then the NOx sensor may not performan SD test. Method 300 then returns.

However, if at 304 it is determined that a key-off event has occurred,then method 300 may proceed to 308, which comprises determining aduration of an after-run routine. An after-run routine may include aduration (e.g., amount of time) for which power to one or more vehiclesystem components (e.g., NOx sensors 190 and 192 shown in FIG. 1B) maycontinue to be supplied after the engine is turned. In some examples,the power may be supplied by a glow plug control module (e.g., glow plugcontrol module 156 shown in FIG. 1B). The duration of the after-runroutine may be determined based on a desired number of SD tests, and aknown duration required to complete one SD test. The duration requiredto complete one SD test may be stored in the memory of the controller.

Then, method 300 may continue to 312, which comprises supplying power tothe NOx sensor for the duration of the after-run routine. In someexamples, method 300 may include supplying power to the NOx sensor fromthe glow plug control module. However, in other examples, method 300 mayinclude supplying power to the NOx sensor from a battery (e.g., battery184 shown in FIG. 1B). Thus, the controller may send signals to one ormore of the glow plug control module and/or the battery to supply powerto the NOx sensor for a duration of the after-run routine. However, inother examples, the controller may send signals to the glow plug controlmodule and/or battery to supply power to the NOx sensor for only aportion of the duration of the after-run routine.

Method 300 may then continue from 312 to 314, which comprises performingone or more SD tests during the duration of the after-run routine. Theperforming of the SD tests may comprise running the NOx sensor in afirst mode where a NOx concentration is estimated and/or measured, andin a second mode, where the oxygen concentration in a second chamber(e.g., second cavity 220 shown in FIG. 2) of the NOx sensor iscontrolled to a higher second level (e.g., 1000 ppm), as described ingreater detail in the method shown in FIG. 4. Thus, FIG. 4 may be asubroutine of the method 300 at 314. In some examples, the method 300 at314 may include the controller sending signals to the NOx control module(e.g., NOx sensor modules 194 and 198 shown in FIG. 1B) for initiatingone or more SD tests at the NOx sensor electrically coupled to thecontrol module. Thus at 314, the NOx sensor may receive signals forperforming one or more SD tests. In some examples, the controller maysend signals to the NOx control module for running one or more SD testsfor the entirety of the after-run routine.

After performing the one or more SD results at 314, the method 300 maycontinue to 316 which comprises receiving outputs (e.g., SD test resultsdata) from the one or more SD tests. Outputs from the SD tests may bereceived from the NOx sensor control module electrically coupled to theNOx sensor. Further, the outputs may be received by the controller viathe CAN bus. The outputs may include measurements and/or estimates ofthe O₂ and/or NOx concentration of the exhaust gas sampled by the NOxsensor during the SD test, tests status, and SD test result. The testsstatus may be an indication of whether the SD test was completed orcancelled. Further, the SD test result, may be one or more pumpingcurrents output by the NOx sensor during the SD test that is compared toa stored reference value and reported as a percent difference from thestored reference value. In some examples, the SD test result may be aratio of a pumping current output by a measuring electrode pair (e.g.,measuring electrode pair 226, 228 shown in FIG. 2) to a stored value.Since the pumping current output by the measuring electrode pair may beused to estimate the NOx concentration, the SD test result may be aratio of the estimated NOx concentration to a stored NOx concentrationvalue. Thus, the method at 314, may include comparing a pumping currentoutput by the NOx sensor during the SD test, and comparing that to avalue stored in memory of the controller.

After receiving the outputs from the one or more SD tests at 316, method300 may then proceed to 318 which comprises excluding outputs from theone or more SD tests received at 316 based on qualification conditions.More specifically, the method 300 at 318 may comprises eliminatingand/or excluding SD tests results based on the qualification conditions,which is explained in greater detail below with reference to FIGS. 5-6.Thus, the methods described in FIGS. 5-6 may be a subroutine of themethod 300 at 318. The qualification conditions may include exhaust gastemperatures, O₂ concentrations, NOx concentrations etc. The firstcompleted SD test result generated by the NOx sensor after the key-offevent may be excluded at 318. Further, if one or more of the exhaust gastemperature is greater than a threshold, NOx concentration is greaterthan a threshold, and oxygen concentration is less than a firstthreshold or greater than a second threshold then that SD test resultmay be excluded. Said another way, tests results from any SD testsperformed where the exhaust gas temperature is greater than a threshold,and/or the oxygen concentration and/or NOx concentration of the exhaustgas is outside a threshold range may be excluded at 318.

It is also important to note that the thresholds for the exhaust gastemperature, O₂ concentration, and NOx concentration may also be basedon the location of NOx sensor within the exhaust system. For example aNOx sensor (e.g., NOx sensor 190 shown in FIG. 1) positioned upstream ofan SCR catalyst (e.g., SCR catalyst 140 shown in FIG. 1) may havedifferent qualification thresholds than a NOx sensor (e.g., NOx sensor192 shown in FIG. 1) positioned downstream of the SCR catalyst. Saidanother way, the threshold temperature range, oxygen concentrationrange, and NOx range used to exclude SD test results may be adjusteddepending on the location of the NOx sensor within the exhaust system.However, in other embodiments, the thresholds may be the same for NOxsensor positioned upstream and/or downstream of the SCR catalyst.

After excluding a portion of the SD tests results at 318 method 300 maythen proceed to 320, which comprises applying a correction factor toonly the SD test results not excluded at 318. The correction factor maybe based on the mean NOx and mean O₂ measured and/or estimated duringthe SD test. Additionally, the correction factor may be stored in alook-up table in memory of the controller. Further, the correctionfactor may be a function of mean O₂ and mean NOx measured and/orestimated during the first mode of operation of the NOx sensor in the SDtest. Thus, during the portion of the SD test, where the O₂ and NOxlevels are estimated, the method 300 may include determining the mean ofthe O₂ and NOx concentrations during this portion of the SD test, andthen applying a correction factor to the SD test result based on themean O₂ and NOx concentrations. As such, the method 300 at 320 mayinclude modifying the SD test result obtained at 316 based on mean NOxand O₂ levels estimated during the SD test from which the SD test resultwas received.

It is also important to note that the correction factor may also bebased on the location of NOx sensor within the exhaust system. Forexample a NOx sensor (e.g., NOx sensor 190 shown in FIG. 1) positionedupstream of an SCR catalyst (e.g., SCR catalyst 140 shown in FIG. 1) mayhave different correction factors than a NOx sensor (e.g., NOx sensor192 shown in FIG. 1) positioned downstream of the SCR catalyst. However,in other embodiments, the correction factor may be the same for NOxsensor positioned upstream and/or downstream of the SCR catalyst.

Method 300 may then continue from 320 to 322 which comprises determiningif the NOx sensor is degraded based on only the SD tests results thatwere not excluded at 318. Said another way, only SD tests results fromcompleted SD tests after a first completed SD test after an enginekey-off event may be used to determine if the NOx sensor is degraded.Further, only SD tests results from completed SD tests where the oxygenconcentration measured during the SD test is greater than a firstthreshold and lower than a second threshold may be used to determine ifthe NOx sensor is degraded. Additionally, only SD test results fromcompleted SD tests where the temperature of the exhaust gasses measuredduring the SD test is less than a threshold may be used to determine ifthe NOx sensor is degraded. In some examples, only SD test results forwhich the mean NOx concentration measured during the SD tests is lessthan a threshold may be used to determine if the NOx sensor is degradedin a way that the sensor is skewed low. The method described in FIG. 5,may be executed to determine if the NOx sensor is degraded in a way thatthe sensor is skewed low. On the other hand, the method described inFIG. 6, may be executed to determine if the NOx sensor is degraded in away that the sensor is skewed high. Thus, in some examples, where method300 includes excluding SD tests results based on NOx concentration(e.g., method 500 shown in FIG. 5), method 300 may only be executed todetect a skewed low type of degradation, and not a skewed high type ofdegradation. However, in other examples, where method 300 does notinclude excluding SD tests results based on NOx concentration (e.g.,method 600 shown in FIG. 6), method 300 may only be executed to detect askewed high type of degradation, and not a skewed low type ofdegradation.

In this way, a method may comprise determining if a NOx sensor isdegraded based only on outputs from a NOx sensor during an SD test, andonly if the outputs are generated from a completed SD test after a firstcompleted SD test after an engine key-off event, where the estimatedand/or measured temperature of exhaust gasses sampled by the NOx sensorand/or flowing past the NOx sensor during the SD test is below athreshold, the estimated and/or measured oxygen concentration of exhaustgasses sampled by the NOx sensor during the SD test is above a firstthreshold and below a second threshold.

Additionally in some examples, determining if the NOx sensor is degradedmay be based only on outputs from a NOx sensor during an SD test, andonly if the outputs are generated from a completed SD test after a firstcompleted SD test after an engine key-off event, where the estimatedand/or measured temperature of exhaust gasses sampled by the NOx sensorand/or flowing past the NOx sensor during the SD test is below athreshold, the estimated and/or measured oxygen concentration of exhaustgasses sampled by the NOx sensor during the SD test is above a firstthreshold and below a second threshold, and where the estimated and/ormeasured NOx concentration of exhaust gasses sampled by the NOx sensorduring the SD test is below a threshold. Put more simply, NOxconcentrations of exhaust gasses sampled by the NOx sensor during the SDtest may in some examples be used to disqualify SD test results.However, in other examples, such as when determining if the NOx sensoris degraded in a skewed low manner, the SD test results may be used todetermine if the NOx sensor is degraded regardless of outputs generatedby the NOx sensor during a portion of the SD test corresponding to NOxconcentrations.

Determining if the NOx sensor is degraded at 322 may include comparingthe SD test result to a lower first threshold, and a higher secondthreshold. If the SD test result is less than the lower first thresholdor higher than the higher second threshold, then it may be determined at322 that the NOx sensor is degraded. The lower first threshold andhigher second threshold may in some example be pre-set values stored inthe memory of the controller. Thus, in some examples, the firstthreshold and second threshold may be constant. However, in otherexamples, the first threshold and second threshold may be based as afunction of mean NOx concentration. As such, the first threshold andsecond thresholds may monotonically increase with increasing mean NOxconcentrations, where the mean NOx concentrations are estimated based onthe outputs received at 316. Specifically, the mean NOx concentrationsreceived at 316 may be NOx concentrations estimated over a duration,where the duration is a portion and/or all of the duration of the SDtest. As such, the mean NOx concentration may be calculated based on aplurality outputs corresponding to a plurality of NOx concentrations,taken over a portion of the duration of the SD test. Further thethreshold may also be based on the location of the NOx sensor in theexhaust system. For example the threshold may be different for a NOxsensor positioned downstream of the SCR catalyst than for a NOx sensorpositioned upstream of the SCR catalyst.

However, in other examples, determining if the NOx sensor is degraded at322 may include comparing the SD test result to a stored referencevalue. As described above the stored reference value may be a function,and may be based on mean NOx concentration. In some examples, where thevalue is a function, the function may monotonically increase withincreasing mean NOx concentration. However, it may be determined thatthe NOx sensor is degraded if the difference from the SD test result andthe stored reference value is greater than a threshold. Said anotherway, the method may comprise determining that the NOx sensor is degradedif the SD test result is different from the stored reference value bymore than a threshold amount.

If it is determined that the NOx SD test result is less than the firstthreshold or greater than the second threshold at 322, then it may bedetermined that the NOx sensor is degraded, and method 300 continues to326 which may comprise alerting the vehicle operator of the NOx sensordegradation. In some examples, the alerting the vehicle operator mayinclude generating a warning light or indicator via a light, LEDdisplay, touch screen display, etc., on a vehicle display and/ordashboard of a vehicle. Method 300 then returns.

However, if it is determined that the SD test result is in-between thefirst and second thresholds, then it may be determined that the NOxsensor is not degraded, and method 300 may proceed from 322 to 324,which comprises not alerting the vehicle operator. Method 300 thenreturns.

In this way, a method may comprise after a vehicle ignition key-offevent: running multiple nitrogen oxide (NOx) sensor self-diagnostictests, where running each test comprises running the NOx sensor in afirst mode and a second mode. Thus a SD test may only be renderedcomplete, if the NOx sensor has been run in both the first mode and thesecond mode. In the first mode, the method may comprise applying a firstelectric potential across an electrode pair of a nitrogen oxide (NOx)sensor so that an oxygen concentration in an internal cavity of the NOxsensor may be adjusted to a lower first level, and generating a firstoutput where the first output is indicative of a NOx concentration. Thesecond mode may comprise applying a second electric potential across theelectrode pair so that an oxygen concentration in an internal cavity ofthe NOx sensor may be adjusted to a higher second level, and generatinga second output. In some examples the second output may be indicative ofan oxygen concentration in the internal cavity.

The method may additionally or alternatively comprise estimating atemperature of exhaust gasses being sampled and/or flowing past the NOxsensor based on outputs from a temperature sensor. In some examples, thetemperature sensor may be aligned with the NOx sensor relative to a flowof exhaust gasses.

As such, the temperature, oxygen concentration, and NOx concentration ofexhaust gasses being sampled and/or flowing past the NOx sensor during aportion and/or all of a duration of each SD test may be estimated. Thetemperature, oxygen concentration, and NOx concentration may betransmitted to a controller via a CAN bus, along with an indication ofwhether each SD test was completed or cancelled. Further, the order inwhich the SD tests occurred after the key-off event may be transmittedto the controller.

The method may further comprise calculating a mean for the oxygenconcentration, NOx concentration, and temperature for each SD test.Additionally or alternatively, the method may comprise determining thatthe sensor is degraded if the first output is different from a referencevalue by more than a threshold, only if the oxygen concentration isgreater than a first threshold and less than a second threshold, thetemperature is less than a threshold, and the SD test was completedafter a first completed SD test following the key-off event.Additionally, the method may comprise determining that the sensor isdegraded if the first output is different from a reference value by morethan a threshold, only if the NOx concentration is less than threshold.

Turning now to FIG. 4, it shows an example method 400 for performing aNOx sensor SD test. Thus, method 400 may be executed at 314 in method300 shown above with reference to FIG. 3. As such, method 400 may beexecuted in conjunction with method 300 shown in FIG. 3. The examplemethod 400 shows how operation of a NOx sensor (e.g., NOx sensor 190,192 shown in FIG. 1B, NOX sensor 200 shown in FIG. 2) may be adjustedduring a SD test. During an SD test, the NOx sensor may operate in twomodes, a first mode and a second mode. In some examples, the NOx sensormay perform the second mode before performing the first mode.

As described above with reference to FIG. 2, the NOx sensor may admit aportion of exhaust gasses flowing in an exhaust system (e.g., exhaustsystem 102 shown in FIG. 1B) to a first chamber (e.g., first cavity 212shown in FIG. 2). Exhaust gasses may then diffuse into a second chamber(e.g., second cavity 220 shown in FIG. 2). Oxygen may be pumped out ofthe first chamber and second chamber by a first pumping cell (e.g.,first pumping electrode pair 214, 216 shown in FIG. 2) and secondpumping cell (e.g., second pumping electrode pair 222, 216 shown in FIG.2), respectively. In the first mode of operation, the oxygenconcentration in the second chamber may be adjusted to a first level(e.g., 10⁻³ ppm). As such, in the first mode, oxygen may be effectivelyremoved from the second chamber, and then an electric potentialsufficient to dissociate NOx molecules may be applied to a measuringelectrode pair (e.g., measuring electrode pair 226, 228 shown in FIG. 2)in the second chamber to measure an amount of NOx in the sampled exhaustgasses. An amount of oxygen in the exhaust gasses may be estimated basedon a pumping current measured at the first pumping cell. Thus, the NOxsensor may be run in the first mode during normal engine operation whenan engine (e.g., engine 110 shown in FIG. 1B) is on (e.g., not after anengine key-off event). As such, the NOx sensor may be run in the firstmode to measure an amount of NOx and/or oxygen in exhaust gasses flowingthrough the exhaust system.

However, during an SD test, the NOx sensor may be operated in the firstmode and a second mode, where in the second mode, the concentration ofoxygen in the second chamber may be maintained at a higher second level(e.g., 1000 ppm). Thus, during the second mode of operation the electricpotential applied to a measuring electrode pair may dissociate theoxygen molecules in the second chamber, and thus the concentration ofoxygen in the second chamber may be measured. Thus, in the second modeof operation, the oxygen concentration in the second chamber may bemeasured.

Instructions for carrying out method 400 may be executed by a controlmodule (e.g., NOx sensor modules 194, 198 shown in FIG. 1B) based oninstructions stored on a memory of the control module and in conjunctionwith signals received from a NOx sensor (e.g., NOx sensors 190 and 192shown in FIG. 1B) and a controller (e.g., controller 112 shown in FIG.1B) via a CAN bus (e.g., CAN bus 152 shown in FIG. 1B). The controllermay employ engine actuators of the engine system to adjust engineoperation, according to the methods described below. Thus, the NOxsensor module may adjust electric voltage applied to the NOx sensorbased on signals received from the controller. Specifically, thecontroller may send a signal to the NOx control module to run one ormore SD tests via the CAN bus. The NOx control module may perform the SDtest, and report outputs from the NOx sensor back to the controller viathe CAN bus. Specifically, the outputs may include signals correspondingto the status of the SD test (e.g., active, complete, incomplete,cancelled, etc.), an SD test result, NOx concentrations, and oxygenconcentrations.

Said another way, one or more of the status of the SD test, a SD testresult, NOx concentrations, and oxygen concentrations may be estimatedbased on outputs from the NOx sensor. As described below, the SD testresult, NOx concentrations, and oxygen concentrations may be estimatedonly based on outputs from the NOx sensor during a portion of the SDtest. Specifically, the SD test result, NOx concentrations, and oxygenconcentrations may only be estimated based on outputs from the NOxsensor during a first mode of the SD test, where a concentration ofoxygen in a cavity of the NOx sensor is adjusted to a lower first level(e.g., 10⁻³ ppm). The NOx concentrations may be an estimatedconcentration of NOx in exhaust gasses sampled by the NOx sensor.Similarly, the oxygen concentrations may be an estimated concentrationof oxygen in exhaust gasses sampled by the NOx sensor. The outputstransmitted to the controller from the NOx sensor and/or control modulemay then be used by the controller to perform a method, such as themethod 300 described above with reference to FIG. 3.

Method 400 begins at 402, which comprises estimating and/or measuringexhaust system operating conditions. Exhaust system operating conditionsmay include an exhaust gas temperature, exhaust gas NOx and/or O₂concentration, etc.

After estimating exhaust system operating parameters, method 400 maycontinue to 404 which comprises determining if an SD test is desired. AnSD test may be desired after an engine key-off event. Thus, the method400 may additionally comprise determining if an engine key-off event hasoccurred in a manner similar that at 304 of method 300 described abovewith reference to FIG. 3. If an engine key-off event has not occurredand/or an SD test is not desired, method 400 may proceed to 406 whichcomprises not performing an SD test. Thus, at 406, an SD test may not beperformed at the NOx sensor. Method 400 then returns. However, if at 404it is determined that an engine key-off event has occurred and an SDtest is desired, then method 400 continues to 408 and runs the NOxsensor in a second mode to control the oxygen concentration in thesecond chamber to a higher second threshold.

In the second mode of operation, a second electric potential (VIp1 shownin FIG. 2) applied across the second pumping cell may be adjusted to asecond level to maintain the oxygen concentration in the second chamberat approximately a higher second threshold. In some examples the secondthreshold may be approximately 1000 ppm of oxygen. In some examples, thesecond mode of operation may additionally include applying a thirdelectric potential across the measuring electrode pair and estimating anoxygen concentration in the second chamber based on the resultingpumping current.

After running the NOx sensor in the second mode, method 400 may thencontinue to 410, and run the NOx sensor in a first mode to control theoxygen concentration in the second chamber to a lower first thresholdand measure NOx and oxygen levels (e.g., concentrations). In the firstmode, a first electric potential (VIp0 shown in FIG. 2) applied acrossthe first pumping cell, and the second electric potential applied acrossthe second pumping cell may be adjusted to remove oxygen from theexhaust gas in the first chamber, and thereby reduce the concentrationof oxygen in the second chamber to a lower first level. In some examplesthe first level may be approximately 10⁻³ ppm of oxygen. The oxygenconcentration of the exhaust gas may also be estimated based on theresulting pumping current (e.g., Ip0 shown in FIG. 2) from the firstelectric potential applied across the first pumping cell. Further, thefirst mode of operation may additionally include adjusting the thirdelectric potential applied across the measuring electrode pair to alevel high enough to dissociate NOx molecules (e.g., 450 mV), andestimating the NOx concentration based on the resulting pumping current.

Thus, the method at 410 may include measuring and/or estimating NOxand/or oxygen levels in the exhaust gas based on outputs of the NOxsensor during an SD test. More specifically, the NOx and/or oxygenconcentrations may be estimated based on outputs from the NOx sensorduring the first mode of operation of the NOx sensor during the SD test.As such, NOx and/or oxygen concentrations may be estimated for aduration, where the duration may be a portion of the duration of an SDtest. Said another way, a completed SD test may last a duration (e.g.,17 seconds). NOx and/or oxygen concentrations may be estimated over aportion of the duration of the SD test. Specifically the oxygen levelsmay be based on a first pumping current (e.g., Ip0 shown in FIG. 2)and/or a second pumping current (e.g., Ip1 shown in FIG. 2) during afirst mode of NOx sensor operation. Additionally, during the first modeof operation, the NOx level may be estimated based on a third pumpingcurrent (e.g., Ip2 shown in FIG. 2). Further, during the first mode ofoperation at 410, an SD test result may be generated. The SD test resultmay be generated based on one or more of the first, second, and thirdpumping currents. In some examples, the SD test result may be based ononly the third pumping current. However, in other examples, the SD testresult may be based on the first pumping current, second pumpingcurrent, and third pumping current. In still further examples, the SDtest result may be based on the second and third pumping currents.

Method 400 may then continue from 410 to 412 which comprises comparingthe SD test result to a stored value. Thus, the one or more pumpingcurrents corresponding to the SD test result at 410, may be compared toa reference value stored in the memory of the controller. The comparingmay include dividing the SD test result by the stored reference value,and reporting the SD test result as a percentage of the stored referencevalue. Thus, if the SD test result is the same as the stored referencevalue, then the SD test result may be reported at 412 as 100%. If the SDtest result is greater than the stored reference value, then the SD testresult may be reported as a percentage greater than 100%. Further, ifthe SD test result is less than the stored reference value, then the SDtest result may be reported as a percentage less than 100%. However, inother examples, the SD test result may be reported as a ratio. Thus, ifthe SD test result is the same as the stored reference value, then theSD test result may be reported at 412 as 1. If the SD test result isgreater than the stored reference value, then the SD test result may bereported as decimal value greater than 1. If the SD test result is lessthan the stored reference value, then the SD test result may be reportedas a decimal value smaller than 1.

The reference value may be different depending on the positioning of theNOx sensor within an exhaust system (e.g., exhaust system 102 shown inFIG. 1B). For example, the reference value may be different for a firstNOx sensor (e.g., NOx sensor 190 shown in FIG. 1B) positioned upstreamof an SCR catalyst (e.g., SCR catalyst 140 shown in FIG. 1B), than for asecond NOx sensor (e.g., NOx sensor 192 shown in FIG. 1B) positioneddownstream of the SCR catalyst.

The method 400 may then proceed from 412 to 414, which comprisestransmitting the oxygen and NOx levels (e.g. concentrations) measuredand/or estimated at 410 and/or the SD test result reported at 410. Insome examples, the method 400 at 414 may include transmitting the oxygenand NOx level measured and/or estimated at 410 and/or the SD test resultreported at 410 to the controller. Thus, at 414, the NOx control modulemay transmit the SD test result, oxygen concentration, and NOxconcentration, measured and/or estimated over the duration of the SDtest performed at 408-412 to the controller via the CAN bus. Method 400then returns.

Turning now to FIG. 5, it shows an example method 500 for excluding SDtests results received from a NOx sensor (e.g. NOx sensor 190, 192 shownin FIG. 1B, NOX sensor 200 shown in FIG. 2) based on a set of qualifyingconditions. Method 500 may be executed as part of a method (e.g., method300 shown in FIG. 3) for determining if the NOx sensor is degraded in away that is skewed low. Thus, a method such as the method 500 may beexecuted as a subroutine of method 300 at 318. Said another way, method500 may be executed as part of a method, such as the method 300 shown inFIG. 3, to determine if a NOx is degraded in a way that is skewed low.As explained above with reference to FIG. 1, the NOx sensor may performone or more SD tests after a key-off event. Results from the NOx sensormay first be transmitted to a NOx sensor control module (e.g., NOxsensor modules 194 and 198 shown in FIG. 1B), before being relayed to acontroller (e.g., controller 112) via a CAN bus (e.g., CAN bus 152 shownin FIG. 1B). Specifically, the NOx sensor control module may send astatus of the SD test (e.g., complete, cancelled, active, etc.), an SDtest result, oxygen concentration, and NOx concentration to thecontroller. The controller may then exclude SD tests results based onthe set of qualifying conditions. A method such as the method 500 shownin FIG. 5, may be performed by the controller to exclude SD testsresults based on the qualification conditions.

Instructions for carrying out method 500 may be executed by a controller(e.g., controller 112 shown in FIG. 1B) based on instructions stored ona memory of the controller and in conjunction with signals received fromvarious sensors of the engine system, such as the sensors describedabove with reference to FIGS. 1A-1B. Specifically, the controller mayexecute method 500 based on outputs from the NOx sensor received fromthe NOx sensor control module. The controller may employ engineactuators of the engine system to adjust engine operation, according tothe methods described below.

Method 500 begins at 502, which comprises estimating and/or measuringengine operating conditions. Engine operating conditions may include anexhaust gas temperature, exhaust gas NOx and/or O₂ concentration, enginetemperature, an intake manifold vacuum, a position of an intake valve, aposition of a throttle valve, etc.

After estimating and/or measuring engine operating conditions, method500 may proceed to 504 which comprises receiving outputs from an SDtest. The outputs received from the SD test may include one or more ofan SD test status, oxygen concentrations, NOx concentrations, and SDtest result. As described above with reference to FIGS. 3 and 4, thetest status may be generated by the NOx sensor control module and mayindicate whether the SD test is complete or incomplete. Further, theoxygen concentrations of the exhaust gas as estimated and/or measured bythe NOx sensor during the SD test may be received at 504. Additionally,the NOx concentration of the exhaust gas as estimated and/or measured bythe NOx sensor during the SD test may be received at 504. Specifically,as described above with reference to FIG. 4, the oxygen concentrationand NOx concentration may be estimated and/or measured during the firstmode of operation of the NOx sensor during the SD test. The SD testresult may be reported as a percentage value compared to a stored valuein the NOx sensor control module as described above with reference toFIG. 4.

After receiving the outputs from the SD test at 504, method 500 mayproceed to 505, 506, 510, 512, and/or 514 in any order. In someexamples, 505-514 may be executed simultaneously. However, in theexample shown in FIG. 5, method 500 may proceed from 504 to 505 whichcomprises determining if the SD test is completed. Said another way, themethod 500 at 505 may comprise determining if the output (e.g., SD testresult) received at 504 is from a completed SD test. The NOx sensorcontrol module may send a signal to the controller indicating whetherthe SD test has been completed, or cancelled. Thus, determining whetheror not the SD test is complete may be based on signals received from theNOx sensor control module.

If the SD test from which the outputs were received at 504, isdetermined at 505 to not be complete, then method 500 may proceed to508, which comprises excluding the outputs from the SD test. Morespecifically, the method 500 at 508 may comprise excluding the SD testresult from the SD test. Thus, if the SD is not completed, then theoutputs received from the NOx sensor for that SD test may be excluded.As described above with reference to FIG. 3, SD tests results and/oroutputs received from the NOx sensor that are excluded at 508 may not beused in determining whether or not the NOx sensor is degraded such as at322 in method 300 in FIG. 3. Thus, the method at 508 comprises excludingSD test results and/or outputs received from the NOx sensor for SD teststhat have not been completed. More simply, if the SD test does not havea completed status signal, then the SD test results for that SD test maybe excluded, and may not be used to determine if the NOx sensor isdegraded.

However, if it is determined that the SD test is complete at 505, thenmethod 500 may proceed to 506 which comprises determining if the meantemperature of exhaust gasses sampled by the NOx sensor and/or flowingpast the NOx sensor during the SD test is less than a threshold. Thethreshold may be stored in memory of the controller. Further thethreshold may be a temperature above which, urea may be converted to NH₃at more than a threshold rate. Thus, the threshold temperature mayrepresent a temperature above, which NH₃ may be registered as NOx by theNOx sensor.

In some examples the mean temperature may only be the mean temperatureof exhaust gasses during the first mode of operation of the NOx sensorduring the SD test. The temperature of exhaust gasses may be estimatedbased on outputs from a temperature sensor (e.g., temperature sensor191, 193 shown in FIG. 1B), aligned with the NOx sensor in an exhaustsystem (e.g., exhaust system 102 shown in FIG. 1B). Thus, as describedabove with reference to FIG. 1B, the temperature sensor may besuperposed with respect to the NOx sensor and thus may be configured tomeasure the temperature of exhaust gasses flowing past and/or beingsampled at the NOx sensor.

If the mean temperature of the exhaust gasses being sampled at the NOxsensor and/or flowing past the sensor during the SD test is not lessthan the threshold at 506, then method 500 proceeds to 508 whichcomprises excluding outputs received from the NOx sensor for that SDtest. Thus, if the mean exhaust gas temperature of exhaust gasses beingsampled at the NOx sensor during all or a portion of the SD test exceedsthe threshold, then the SD test result and/or outputs received from theNOx sensor for that SD test may be excluded. As described above withreference to FIG. 3, SD tests results and/or outputs received from theNOX sensor that are excluded at 508 may not be used in determiningwhether or not the NOx sensor is degraded such as at 322 in method 300in FIG. 3. Thus, the method at 508 comprises excluding outputs receivedfrom the NOx sensor from SD tests where the estimated and/or measuredtemperature of exhaust gasses sampled by the NOx sensor and/or flowingpast the NOx sensor during the SD test is above a threshold. Moresimply, if the exhaust gas temperature as estimated and/or measuredbased on outputs of the temperature sensor arranged in line with the NOxsensor relative to the flow of exhaust gasses during an SD test isgreater than a threshold, then the SD test results for that SD test maybe excluded, and may not be used to determine if the NOx sensor isdegraded.

Returning to 506, if it is determined that the mean temperature ofexhaust gasses is less than the threshold at 506, then method 500 maycontinue to 510, which comprises determining if the oxygen concentrationof exhaust gasses estimated and/or measured during the first mode ofoperation of the SD test, is greater than a lower first threshold andless than a higher second threshold. The first and second threshold maybe stored in the memory of the controller. As described above withreference to FIG. 4, the oxygen concentration may be estimated based onone or more of a first pumping current (e.g., Ip0) and second pumpingcurrent (e.g., Ip1) from a first pumping cell (e.g., first pumpingelectrode pair 214, 216 shown in FIG. 2) and second pumping cell (e.g.,second pumping electrode pair 222, 216 shown in FIG. 2), respectively.Further, as described above with reference to FIG. 4, the oxygenconcentration may be estimated during a first mode of operation of theNOx sensor during the SD test, where the oxygen concentration in asecond chamber (e.g., second internal cavity 220 shown in FIG. 2) of theNOx sensor is reduced from that of a first chamber (e.g., first internalcavity 212 shown in FIG. 2) to a lower first level (e.g., 10⁻³ ppm). Ifthe oxygen concentration is less than the first threshold, or greaterthan the second threshold, then method 500 may continue to 508 andexclude the outputs from the SD test.

Thus, the method at 508 comprises excluding outputs (e.g., SD testresults) from SD tests where the estimated and/or measured oxygenconcentration of exhaust gasses sampled by the NOx sensor during the SDtest is below a first threshold or above a second threshold. Moresimply, if the oxygen concentration as estimated and/or measured basedon outputs of the NOx sensor during an SD test is less than a firstthreshold or greater than a second threshold, then the outputs and/or SDtest results received from the NOx sensor for that SD test may beexcluded, and may not be used to determine if the NOx sensor isdegraded.

However, if the oxygen concentration is between the first and secondthreshold at 510, then method 500 may continue to 512, which comprisesdetermining if the outputs (e.g., SD test results) received from the NOxsensor are from a first completed SD test after an engine key-off event.As described above with reference to FIG. 3, multiple SD tests may beperformed after an engine key-off event. However, the method 500 at 510may include determining if the outputs received from the NOx sensor arefrom the first SD test completed after an engine key-off event, so thatresults from the first completed SD test after the engine key-off eventmay be excluded.

If it is determined at 512, that the outputs (e.g., SD test results) arefrom the first SD test completed after an engine key-off event, thenmethod 500 may continue to 508 and exclude the outputs (e.g., SD testresults). Thus, the method 500 at 508 may comprise only including thesecond and subsequent SD tests completed after an engine key-off event,and not including the first SD test completed after an engine key-offevent. For example, if after an engine key-off event, the NOx sensorfirst completes a first SD test, and then completes a second SD testbefore power to the NOx sensor ceases, or the controller stops sendingsignals to the NOx sensor control module to perform additional SD tests,then only outputs from the NOx sensor during the second SD test may beused to determine if the NOx sensor is degraded. However, if at 512 itis determined that the outputs (e.g., SD test results) are not from thefirst completed SD test after an engine key-off event, then method 500may continue to 514 which comprises determining if the NOx concentrationis less than a threshold.

As described above with reference to FIG. 4, the NOx concentration maybe estimated based on outputs from the NOx sensor during the first modeof operation during an SD test. Specifically, the NOx concentration maybe estimated based on a pumping current (e.g., Ip2 shown in FIG. 2) froma measuring electrode pair (e.g., measuring electrode pair 226, 228shown in FIG. 2). The threshold may be a NOx level (e.g., concentration)stored in the memory of the controller. If it is determined at 514 thatthe NOx concentration is not less than the threshold, method 500continues to 508 which comprises excluding the outputs from the NOxsensor during that SD test. Thus, the method at 508 comprises excludingSD test results from SD tests where the estimated and/or measured NOxconcentration of exhaust gasses sampled by the NOx sensor during the SDtest is above a threshold. More simply, if the NOx concentration asestimated and/or measured based on outputs of the NOx sensor during anSD test is greater than a threshold, then the SD test results for thatSD test may be excluded, and may not be used to determine if the NOxsensor is degraded.

However, if at 514 it is determined that the NOx concentration is lessthan the threshold at 514, then method 500 continues to 516 whichcomprises not excluding the SD test result, and using the SD test resultto determine if the NOx sensor is degraded, such as in the mannerdescribed above with reference to 322 of method 300 in FIG. 3. Method500 then returns.

Thus, the method 500 at 508 comprises excluding SD test results from SDtests where the estimated and/or measured temperature of exhaust gassessampled by the NOx sensor and/or flowing past the NOx sensor during theSD test is above a threshold, SD test results from SD tests where theestimated and/or measured oxygen concentration of exhaust gasses sampledby the NOx sensor during the SD test is below a first threshold or abovea second threshold, SD test results from the first SD test completedafter an engine key-off event, and SD test results from SD tests wherethe estimated and/or measured NOx concentration of exhaust gassessampled by the NOx sensor during the SD test is above a threshold.

As such, the method 500 at 516 comprises only including SD tests resultsfrom SD tests where the estimated and/or measured temperature of exhaustgasses sampled by the NOx sensor and/or flowing past the NOx sensorduring the SD test is below a threshold, SD test results from SD testswhere the estimated and/or measured oxygen concentration of exhaustgasses sampled by the NOx sensor during the SD test is above a firstthreshold and below a second threshold, SD test results from the firstSD test completed after a first completed SD test after an enginekey-off event, and SD test results from SD tests where the estimatedand/or measured NOx concentration of exhaust gasses sampled by the NOxsensor during the SD test is below a threshold.

Turning now to FIG. 6, it shows an example method 600 for excluding SDtests results received from a NOx sensor (e.g. NOx sensor 190, 192 shownin FIG. 1B, NOX sensor 200 shown in FIG. 2) based on a set of qualifyingconditions. Method 600 may be executed as part of a method (e.g., method300 shown in FIG. 3) for determining if the NOx sensor is degraded in away that is skewed high. Thus, a method such as the method 600 may beexecuted as a subroutine of method 300 at 318. Said another way, method600 may be executed as part of a method, such as the method 300 shown inFIG. 3, to determine if a NOx is degraded in a way that is skewed high.

While FIG. 5 shows a method for determining if the NOx sensor isdegraded in a way that is skewed low, the method 600 described in FIG. 6may be executed to determine if the NOx sensor is degraded in a way thatis skewed high. Thus, the methods described in FIGS. 5-6 may in someexamples, both be executed, to determine if the NOx sensor is degradedin a way that is skewed low or skewed high. While the method in FIG. 5includes excluding SD tests results received from the NOx sensor if theNOx concentrations as estimated from outputs of the NOx sensor duringthe SD test are below a threshold, the method 600 described in FIG. 6does not include excluding SD tests results based on the NOxconcentrations of exhaust gasses sampled at the NOx sensor during an SDtest.

As explained above with reference to FIG. 1, the NOx sensor may performone or more SD tests after a key-off event. Results from the NOx sensormay first be transmitted to a NOx sensor control module (e.g., NOxsensor modules 194 and 198 shown in FIG. 1B), before being relayed to acontroller (e.g., controller 112) via a CAN bus (e.g., CAN bus 152 shownin FIG. 1B). Specifically, the NOx sensor control module may send astatus of the SD test (e.g., complete, cancelled, active, etc.), an SDtest result, oxygen concentration, and NOx concentration to thecontroller. The controller may then exclude SD tests results based onthe set of qualifying conditions. A method such as the method 600 shownin FIG. 6, may be performed by the controller to exclude SD testsresults based on the qualification conditions.

Instructions for carrying out method 600 may be executed by a controller(e.g., controller 112 shown in FIG. 1B) based on instructions stored ona memory of the controller and in conjunction with signals received fromvarious sensors of the engine system, such as the sensors describedabove with reference to FIGS. 1A-1B. Specifically, the controller mayexecute method 600 based on outputs from the NOx sensor received fromthe NOx sensor control module. The controller may employ engineactuators of the engine system to adjust engine operation, according tothe methods described below.

Method 600 begins at 602, which comprises estimating and/or measuringengine operating conditions. Engine operating conditions may include anexhaust gas temperature, exhaust gas NOx and/or O₂ concentration, enginetemperature, an intake manifold vacuum, a position of an intake valve, aposition of a throttle valve, etc.

After estimating and/or measuring engine operating conditions, method600 may proceed to 604 which comprises receiving outputs from an SDtest. The outputs received from the SD test may include one or more ofan SD test status, oxygen concentrations, NOx concentrations, and SDtest result. As described above with reference to FIGS. 3 and 4, thetest status may be generated by the NOx sensor control module and mayindicate whether the SD test is complete or incomplete. Further, theoxygen concentrations of the exhaust gas as estimated and/or measured bythe NOx sensor during the SD test may be received at 604. Additionally,the NOx concentration of the exhaust gas as estimated and/or measured bythe NOx sensor during the SD test may be received at 604. Specifically,as described above with reference to FIG. 4, the oxygen concentrationand NOx concentration may be estimated and/or measured during the firstmode of operation of the NOx sensor during the SD test. The SD testresult may be reported as a percentage value compared to a stored valuein the NOx sensor control module as described above with reference toFIG. 4.

After receiving the outputs from the SD test at 604, method 600 mayproceed to 605, 606, 610, and/or 612, in any order. In some examples,605-612 may be executed simultaneously. However, in the example shown inFIG. 6, method 600 may proceed from 604 to 605 which comprisesdetermining if the SD test is completed. Said another way, the method600 at 605 may comprise determining if the output (e.g., SD test result)received at 604 is from a completed SD test. The NOx sensor controlmodule may send a signal to the controller indicating whether the SDtest has been completed, or cancelled. Thus, determining whether or notthe SD test is complete may be based on signals received from the NOxsensor control module.

If the SD test from which the outputs were received at 604, isdetermined at 605 to not be complete, then method 600 may proceed to608, which comprises excluding the outputs from the SD test. Morespecifically, the method 600 at 608 may comprise excluding the SD testresult from the SD test. Thus, if the SD is not completed, then theoutputs received from the NOx sensor for that SD test may be excluded.As described above with reference to FIG. 3, SD tests results and/oroutputs received from the NOx sensor that are excluded at 608 may not beused in determining whether or not the NOx sensor is degraded such as at322 in method 300 in FIG. 3. Thus, the method at 608 comprises excludingSD test results and/or outputs received from the NOx sensor for SD teststhat have not been completed. More simply, if the SD test does not havea completed status signal, then the SD test results for that SD test maybe excluded, and may not be used to determine if the NOx sensor isdegraded.

However, if it is determined that the SD test is complete at 605, thenmethod 600 may proceed to 606 which comprises determining if the meantemperature of exhaust gasses sampled by the NOx sensor and/or flowingpast the NOx sensor during the SD test is less than a threshold. Thethreshold may be stored in memory of the controller. Further thethreshold may be a temperature above which, urea may be converted to NH₃at more than a threshold rate. Thus, the threshold temperature mayrepresent a temperature above, which NH₃ may be registered as NOx by theNOx sensor.

In some examples the mean temperature may only be the mean temperatureof exhaust gasses during the first mode of operation of the NOx sensorduring the SD test. The temperature of exhaust gasses may be estimatedbased on outputs from a temperature sensor (e.g., temperature sensor191, 193 shown in FIG. 1B), aligned with the NOx sensor in an exhaustsystem (e.g., exhaust system 102 shown in FIG. 1B). Thus, as describedabove with reference to FIG. 1B, the temperature sensor may besuperposed with respect to the NOx sensor and thus may be configured tomeasure the temperature of exhaust gasses flowing past and/or beingsampled at the NOx sensor.

If the mean temperature of the exhaust gasses being sampled at the NOxsensor and/or flowing past the sensor during the SD test is not lessthan the threshold at 606, then method 600 proceeds to 608 whichcomprises excluding outputs received from the NOx sensor for that SDtest. Thus, if the mean exhaust gas temperature of exhaust gasses beingsampled at the NOx sensor during all or a portion of the SD test exceedsthe threshold, then the SD test result and/or outputs received from theNOx sensor for that SD test may be excluded. As described above withreference to FIG. 3, SD tests results and/or outputs received from theNOX sensor that are excluded at 608 may not be used in determiningwhether or not the NOx sensor is degraded such as at 322 in method 300in FIG. 3. Thus, the method at 608 comprises excluding outputs receivedfrom the NOx sensor from SD tests where the estimated and/or measuredtemperature of exhaust gasses sampled by the NOx sensor and/or flowingpast the NOx sensor during the SD test is above a threshold. Moresimply, if the exhaust gas temperature as estimated and/or measuredbased on outputs of the temperature sensor arranged in line with the NOxsensor relative to the flow of exhaust gasses during an SD test isgreater than a threshold, then the SD test results for that SD test maybe excluded, and may not be used to determine if the NOx sensor isdegraded.

Returning to 606, if it is determined that the mean temperature ofexhaust gasses is less than the threshold at 606, then method 600 maycontinue to 610, which comprises determining if the oxygen concentrationof exhaust gasses estimated and/or measured during the first mode ofoperation of the SD test, is greater than a lower first threshold andless than a higher second threshold. The first and second threshold maybe stored in the memory of the controller. As described above withreference to FIG. 4, the oxygen concentration may be estimated based onone or more of a first pumping current (e.g., Ip0) and second pumpingcurrent (e.g., Ip1) from a first pumping cell (e.g., first pumpingelectrode pair 214, 216 shown in FIG. 2) and second pumping cell (e.g.,second pumping electrode pair 222, 216 shown in FIG. 2), respectively.Further, as described above with reference to FIG. 4, the oxygenconcentration may be estimated during a first mode of operation of theNOx sensor during the SD test, where the oxygen concentration in asecond chamber (e.g., second internal cavity 220 shown in FIG. 2) of theNOx sensor is reduced from that of a first chamber (e.g., first internalcavity 212 shown in FIG. 2) to a lower first level (e.g., 10⁻³ ppm). Ifthe oxygen concentration is less than the first threshold, or greaterthan the second threshold, then method 600 may continue to 608 andexclude the outputs from the SD test.

Thus, the method at 608 comprises excluding outputs (e.g., SD testresults) from SD tests where the estimated and/or measured oxygenconcentration of exhaust gasses sampled by the NOx sensor during the SDtest is below a first threshold or above a second threshold. Moresimply, if the oxygen concentration as estimated and/or measured basedon outputs of the NOx sensor during an SD test is less than a firstthreshold or greater than a second threshold, then the outputs and/or SDtest results received from the NOx sensor for that SD test may beexcluded, and may not be used to determine if the NOx sensor isdegraded.

However, if the oxygen concentration is between the first and secondthreshold at 610, then method 600 may continue to 612, which comprisesdetermining if the outputs (e.g., SD test results) received from the NOxsensor are from a first completed SD test after an engine key-off event.As described above with reference to FIG. 3, multiple SD tests may beperformed after an engine key-off event. However, the method 600 at 610may include determining if the outputs received from the NOx sensor arefrom the first SD test completed after an engine key-off event, so thatresults from the first completed SD test after the engine key-off eventmay be excluded.

If it is determined at 612, that the outputs (e.g., SD test results) arefrom the first SD test completed after an engine key-off event, thenmethod 600 may continue to 608 and exclude the outputs (e.g., SD testresults). Thus, the method 600 at 608 may comprise only including thesecond and subsequent SD tests completed after an engine key-off event,and not including the first SD test completed after an engine key-offevent. For example, if after an engine key-off event, the NOx sensorfirst completes a first SD test, and then completes a second SD testbefore power to the NOx sensor ceases, or the controller stops sendingsignals to the NOx sensor control module to perform additional SD tests,then only outputs from the NOx sensor during the second SD test may beused to determine if the NOx sensor is degraded. However, if at 612 itis determined that the outputs (e.g., SD test results) are not from thefirst completed SD test after an engine key-off event, then method 600may continue to 616 which comprises not excluding the SD test result,and using the SD test result to determine if the NOx sensor is degraded,such as in the manner described above with reference to 322 of method300 in FIG. 3. Method 600 then returns.

Thus, the method 600 at 608 comprises excluding SD test results from SDtests where the estimated and/or measured temperature of exhaust gassessampled by the NOx sensor and/or flowing past the NOx sensor during theSD test is above a threshold, SD test results from SD tests where theestimated and/or measured oxygen concentration of exhaust gasses sampledby the NOx sensor during the SD test is below a first threshold or abovea second threshold, and SD test results from the first SD test completedafter an engine key-off event. Further, the method 600 may not compriseexcluding SD test results from SD tests where the estimated and/ormeasured NOx concentration of exhaust gasses sampled by the NOx sensorduring the SD test is above a threshold.

As such, the method 600 at 616 comprises only including SD tests resultsfrom SD tests where the estimated and/or measured temperature of exhaustgasses sampled by the NOx sensor and/or flowing past the NOx sensorduring the SD test is below a threshold, SD test results from SD testswhere the estimated and/or measured oxygen concentration of exhaustgasses sampled by the NOx sensor during the SD test is above a firstthreshold and below a second threshold, and SD test results from thefirst SD test completed after a first completed SD test after an enginekey-off event.

In this way, the method may comprise excluding SD test results based ona set of qualification conditions. SD results that are excluded based onthe qualification conditions may not be used to determine if the NOxsensor is degraded. Thus, SD tests results from SD tests that do notmeet the qualification conditions may not be used to determine if theNOx sensor is degraded. The qualification conditions may include: SDtests results from completed SD tests after a first completed SD testafter an engine key-off event, SD test results SD tests where themeasured and/or estimated oxygen concentration of the exhaust gassessampled by the NOx sensor during the SD test is between a lower firstand higher second thresholds, and SD test results from SD tests wherethe measured and/or estimated temperature of exhaust gasses sampled bythe NOx sensor during the SD test and/or flowing past the NOx sensorduring the SD test is below a threshold. Further, in some examples, thequalification conditions may additionally include: SD tests resultswhere the measured and/or estimated NOx concentration of exhaust gassessampled by the NOx sensor during the SD test is below a threshold.

Thus, SD tests results from the first completed SD test result after anengine key-off event may be excluded from determining if the NOx sensoris degraded. Further, SD test results may be excluded if the estimatedand/or measured temperature of exhaust gasses sampled by the NOx sensorand/or flowing past the NOx sensor during the SD test is above athreshold. SD test results from SD tests where the estimated and/ormeasured oxygen concentration of exhaust gasses sampled by the NOxsensor during the SD test is below a first threshold or above a secondthreshold may be excluded. Additionally or alternatively, the SD testresults from SD tests where the estimated and/or measured NOxconcentration of exhaust gasses sampled by the NOx sensor during the SDtest is above a threshold may also be excluded.

In this way, a method may comprise determining if a NOx sensor isdegraded based only on outputs from a NOx sensor during an SD test, andonly if the outputs are generated from a completed SD test after a firstcompleted SD test after an engine key-off event, where the estimatedand/or measured temperature of exhaust gasses sampled by the NOx sensorand/or flowing past the NOx sensor during the SD test is below athreshold, the estimated and/or measured oxygen concentration of exhaustgasses sampled by the NOx sensor during the SD test is above a firstthreshold and below a second threshold, and in some examples, where theestimated and/or measured NOx concentration of exhaust gasses sampled bythe NOx sensor during the SD test is below a threshold. Determining ifthe NOx sensor is degraded may include comparing a test result from theNOx sensor generated during a first mode of operation of the NOx sensorduring the SD test to a lower first threshold, and a higher secondthreshold.

If the test result is greater than the higher second threshold or lessthan the lower first threshold, then it may be determined that the NOxsensor is degraded, and/or a vehicle operator may be notified. The firstmode of operation of the NOX sensor may comprise reducing the oxygenconcentration of exhaust gasses in a first chamber of the NOX sensorfrom a higher first level, to a lower second level in the secondchamber. In some examples, the lower second level may be approximately10⁻³ ppm. As such, in the first mode of operation of the NOx sensor, theconcentration of NOx in the exhaust gasses may be measured and/orestimated based on a pumping current from a measuring electrode pair,where the electric potential applied across the measuring electrode pairmay be sufficient to dissociate NOx molecules.

Thus, only qualified NOx sensor outputs, where the qualified NOx sensoroutputs meet one or more qualification conditions, may be used todetermine the NOx sensor is degraded. The qualification conditions mayinclude a completed SD test after a first completed SD test after anengine-key off event, a temperature of exhaust gasses being below athreshold, a concentration of oxygen in the exhaust gasses beingin-between lower first and higher second thresholds, and in someexamples, a concentration of NOx in the exhaust gasses being lower thana threshold.

During engine operation where an engine is on, a reductant such as ureamay be injected into an exhaust system upstream of an SCR catalyst.Together, the reductant and SCR catalyst may chemically reduce NOxmolecules and therefore reduce NOx emissions. However, the urea injectedduring engine use and also urea droplets produced from a delivery linepurging process may remain in the exhaust system after an engine key-offevent. At exhaust temperatures above a threshold, the urea may beconverted to NH₃. Ammonia may be registered by a NOx sensor in theexhaust system as NOx. Therefore, NOx levels estimated based on outputsfrom the NOx sensor may be overestimated when exhaust temperaturesand/or urea levels exceed respective threshold levels. Further, becauseestimations and/or measurements of the NOx may vary due to differentlevels of urea and exhaust temperatures, NOx sensor that are degradedmay not be diagnosed.

Specifically, NOx sensor may run a self-diagnostic (SD) test to in orderto detect sensor degradation. However, high levels of urea and/orexhaust temperatures may reduce the accuracy of SD tests. Specifically,the ammonia produced by urea and high exhaust temperatures may beregistered by the NOx sensor as NOx during the SD test. Accordingly,test results from separate SD tests of a NOx sensor that is not degradedmay be as much or more different from one another than test results fromSD tests of a NOx that is not degraded and test results from SD tests ofa NOx sensor that is degraded. Therefore, NOx sensors that are degradedmay not be distinguished and identified from NOx sensors that are notdegraded. If a degraded NOx sensor is not identified, the accuracy ofestimations of NOx levels may be reduced. As a result, degradation ofthe SCR catalyst and/or other components in the exhaust system used tocontrol NOx emissions may fail to be detected, leading to increased NOxemissions.

However, a method may comprise determining if a NOx sensor is degradedbased only on outputs from a NOx sensor during an SD test if atemperature of exhaust gasses is less than a threshold, and aconcentration of NOx of the exhaust gasses is less than a threshold.

In this way, a technical effect of increasing the sensitivity andaccuracy of detection of NOx sensor degradation is achieved by a methodfor determining if a NOx sensor is degraded based only on outputs from aNOx sensor during an SD test, and only if the outputs are generated froma completed SD test after a first completed SD test after an enginekey-off event, where the estimated and/or measured temperature ofexhaust gasses sampled by the NOx sensor and/or flowing past the NOxsensor during the SD test is below a threshold, the estimated and/ormeasured oxygen concentration of exhaust gasses sampled by the NOxsensor during the SD test is above a first threshold and below a secondthreshold, and the estimated and/or measured NOx concentration ofexhaust gasses sampled by the NOx sensor during the SD test is below athreshold.

By excluding SD test results from SD tests where the exhaust gastemperature is greater than a threshold, and/or the oxygen concentrationis outside the threshold range, and/or the NOx concentration is greaterthan a threshold, and/or the SD test is a first SD test after an enginekey-off event, the variance in the SD test results may be reduced. Assuch, the sensitivity for distinguishing a degraded NOx sensor from aNOx sensor that is not degraded may be increased. Said another way, inthe methods described herein, a higher percentage of NOx sensors thathave become degraded may be detected than methods not excluding SD testsresults based on exhaust temperature, NOx concentration, oxygenconcentration, etc. Put more simply, a technical effect of improving theaccuracy of outputs from a NOx sensor during a SD test is achieved byexcluding SD test results in the manner described above. Thus, byimproving the accuracy of SD test results, the efficiency in detecting adegraded NOx sensor may be increased. Therefore, the efficiency of a NOxemission control system in an exhaust system may be increased.

In this way, method may comprise: determining that a nitrogen oxide(NOx) sensor is degraded based on outputs received from the sensor via aCAN bus during a self-diagnostic (SD) test performed after a firstcompleted SD test after a key-off event, only if the outputs aregenerated under conditions where a temperature at the sensor is lessthan a threshold, a NOx concentration is less than a threshold, and anoxygen concentration is within a threshold range. In some examples, thekey-off event may comprise terminating a combustion cycle in an enginebased on input from a vehicle operator via an input device. The outputsreceived from the NOx sensor, may in some examples be received via theCAN bus from a NOx control module, where the NOx control module may bein electrical communication with the NOx sensor.

Additionally, in some examples, the NOx concentration and oxygenconcentration may be estimated based on outputs from the NOx sensorduring the SD test. The temperature at the sensor may be based onoutputs from a temperature sensor positioned in line with the NOx sensorrelative to a flow of exhaust gasses past the NOx sensor. Additionallyor alternatively, the method may comprise determining that the NOxsensor is degraded if the outputs of the SD test are not within athreshold range. In some examples, the threshold range may be determinedbased on the positioning of the NOx sensor relative to a selectivecatalytic reduction (SCR) catalyst. The outputs received from the NOxsensor may include SD tests results corresponding to the SD test, andwhere the SD test results may be a ratio between one or more pumpingcurrents output by the NOx sensor and stored values. The method may insome examples additionally or alternatively comprise alerting a vehicledriver if it is determined that the NOx sensor is degraded. A completedSD test may comprise running the NOx sensor in both a first mode and asecond mode, the second mode before the first mode, wherein the firstmode may comprise adjusting the oxygen concentration in a cavity of theNOx sensor to a first level and measuring a concentration of NOx in thecavity, and wherein the second mode comprises adjusting the oxygenconcentration in the cavity of the NOx sensor to a second level and notmeasuring a concentration of NOx in the cavity.

In another representation, a method may comprise: excluding a firstcompleted self-diagnostic (SD) test result of a NOx sensor after anengine key-off event, excluding test results from a completed SD test ifone or more of an exhaust gas temperature is greater than a threshold,an oxygen concentration of the exhaust gas is outside a threshold range,and a NOx concentration of the exhaust gas is higher than a threshold,otherwise not excluding test results from a completed SD test; anddetermining that the sensor is degraded only if the non-excluded testresults are different from a reference value by more than a threshold.In some examples, the method may further comprise determining that an SDtest is completed based on outputs from a NOx control module via a CANbus, where the NOx control module may be in electrical communicationwith the NOx sensor. The exhaust gas temperature may be a temperature ofexhaust gasses flowing past the NOx sensor during the SD test, and thetemperature may be estimated based on outputs from a temperature sensorsuperposed with respect to the NOx sensor.

Additionally, the oxygen concentration and NOx concentration of theexhaust gas may in some examples be estimated based on outputs from theNOx sensor during the SD test. The method may additionally oralternatively comprise applying a correction factor to the test resultsbased on a mean NOx concentration of the exhaust gas, where the mean NOxconcentration is calculated based on the NOx concentration of theexhaust gas estimated over a duration, the duration being a portion ofthe SD test. In some examples, the method may additionally comprisesupplying power to the NOx sensor from a glow plug control module, afterthe engine key-off event. Additionally or alternatively, the method maycomprise receiving outputs from the NOx sensor via a NOx control module,where the outputs may comprise one or more of a status of an SD test, anSD test result, the oxygen concentration, and the NOx concentration. Themethod of claim 16, wherein the status of the SD test indicates whetheror not the SD test has been completed.

In another representation, a system may comprise: a first NOx sensorpositioned in an engine exhaust system upstream of a selective catalyticreduction (SCR) catalyst, a second NOx sensor positioned downstream ofthe SCR, a first temperature sensor aligned with the first NOx sensorrelative to an exhaust gas flow in the exhaust system for measuring atemperature of exhaust gasses flowing past the first NOx sensor, asecond temperature sensor aligned with the second NOx sensor relative tothe exhaust gas flow in the exhaust system for measuring a temperatureof exhaust gasses flowing past the second NOx sensor, and a controllerin electrical communication with the first and second NOx sensor via aCAN bus, the controller having computer-readable instructions. Thecomputer-readable may include instruction for determining that one ormore of the first NOx sensor and second NOx sensor are degraded based onoutputs from the first NOx and temperature sensor, and the second NOxand temperature sensors, respectively, where the outputs may begenerated during a self-diagnostic test after a vehicle ignitionkey-off. A test result, test status, and one or more NOx concentrationsand oxygen concentrations of exhaust gasses sampled by the NOx sensorsmay be estimated based on the outputs received from NOx sensors, and atemperature of exhaust gases may be estimated based on outputs receivedfrom the temperature sensors.

Note that the example control and estimation routines included hereincan be used with various engine and/or vehicle system configurations.The specific routines described herein may represent one or more of anynumber of processing strategies such as event-driven, interrupt-driven,multi-tasking, multi-threading, and the like. As such, various acts,operations, or functions illustrated may be performed in the sequenceillustrated, in parallel, or in some cases omitted. Likewise, the orderof processing is not necessarily required to achieve the features andadvantages of the example embodiments described herein, but is providedfor ease of illustration and description. One or more of the illustratedacts or functions may be repeatedly performed depending on theparticular strategy being used. Further, the described acts maygraphically represent code to be programmed into the computer readablestorage medium in the engine control system.

It will be appreciated that the configurations and routines disclosedherein are exemplary in nature, and that these specific embodiments arenot to be considered in a limiting sense, because numerous variationsare possible. For example, the above technology can be applied to V-6,I-4, I-6, V-12, opposed 4, and other engine types. The subject matter ofthe present disclosure includes all novel and nonobvious combinationsand subcombinations of the various systems and configurations, and otherfeatures, functions, and/or properties disclosed herein.

The following claims particularly point out certain combinations andsubcombinations regarded as novel and nonobvious. These claims may referto “an” element or “a first” element or the equivalent thereof. Suchclaims should be understood to include incorporation of one or more suchelements, neither requiring nor excluding two or more such elements.Other combinations and subcombinations of the disclosed features,functions, elements, and/or properties may be claimed through amendmentof the present claims or through presentation of new claims in this or arelated application.

Such claims, whether broader, narrower, equal, or different in scope tothe original claims, also are regarded as included within the subjectmatter of the present disclosure.

1. A method comprising: determining that a nitrogen oxide (NOx) sensoris degraded based on outputs received from the sensor during aself-diagnostic (SD) test performed after a first completed SD testafter a key-off event, only if the outputs are generated underconditions where a temperature at the sensor is less than a threshold,and an oxygen concentration is within a threshold range.
 2. The methodrecited in claim 1, wherein the determining that the NOx sensor isdegraded is based further on outputs received from the sensor, only ifthe outputs are generated under conditions where a NOx concentration isless than a threshold.
 3. The method recited in claim 1, wherein thekey-off event comprises terminating a combustion cycle in an enginebased on input from a vehicle operator via an input device.
 4. Themethod recited in claim 1, wherein the outputs received from the NOxsensor, are received via a CAN bus from a NOx control module, where theNOx control module is in electrical communication with the NOx sensor.5. The method recited in claim 1, wherein the NOx concentration andoxygen concentration are estimated based on outputs from the NOx sensorduring the SD test.
 6. The method recited in claim 1, wherein thetemperature at the sensor is based on outputs from a temperature sensorpositioned in line with the NOx sensor relative to a flow of exhaustgasses past the NOx sensor.
 7. The method recited in claim 1, furthercomprising determining that the NOx sensor is degraded in response tothe received outputs of the SD test being outside a threshold range. 8.The method recited in claim 6, where the threshold range is determinedbased on the positioning of the NOx sensor relative to a selectivecatalytic reduction (SCR) catalyst.
 9. The method recited in claim 1,wherein the outputs received from the NOx sensor are SD tests resultscorresponding to the SD test, and where the SD test results are a ratiobetween one or more pumping currents output by the NOx sensor and storedvalues.
 10. The method recited in claim 1, further comprising alerting avehicle driver in response to determining that the NOx sensor isdegraded.
 11. The method recited in claim 1, wherein the completed SDtest comprises running the NOx sensor in both a first mode and a secondmode, the second mode before the first mode, wherein the first modecomprises adjusting the oxygen concentration in a cavity of the NOxsensor to a first level and measuring a concentration of NOx in thecavity, and wherein the second mode comprises adjusting the oxygenconcentration in the cavity of the NOx sensor to a second level and notmeasuring a concentration of NOx in the cavity.
 12. A method comprising:excluding a first completed self-diagnostic (SD) test result of a NOxsensor after an engine key-off event; excluding test results from acompleted SD test if one or more of an exhaust gas temperature isgreater than a threshold, and an oxygen concentration of the exhaust gasis outside a threshold range; otherwise not excluding test results froma completed SD test; and determining that the sensor is degraded only ifthe non-excluded test results are different from a reference value bymore than a threshold.
 13. The method of claim 12, further comprisingdetermining that an SD test is completed based on outputs from a NOxcontrol module via a CAN bus, where the NOx control module is inelectrical communication with the NOx sensor.
 14. The method of claim10, wherein the exhaust gas temperature is a temperature of exhaustgasses flowing past the NOx sensor during the SD test, and where thetemperature is estimated based on outputs from a temperature sensorpositioned adjacent to the NOx sensor.
 15. The method of claim 10,wherein the oxygen concentration of the exhaust gas is estimated basedon outputs from the NOx sensor during the SD test.
 16. The method ofclaim 12, further comprising applying a correction factor to the testresults based on a mean NOx concentration of the exhaust gas, where themean NOx concentration is calculated based on a NOx concentration of theexhaust gas estimated over a duration, the duration being a portion ofthe SD test.
 17. The method of claim 10, further comprising, supplyingpower to the NOx sensor from a glow plug control module, after theengine key-off event.
 18. The method of claim 10, further comprisingreceiving outputs from the NOx sensor via a NOx control module, wherethe outputs comprise one or more of a status of an SD test, where thestatus of the SD test indicates if the SD test is completed, an SD testresult, the oxygen concentration, and a NOx concentration.
 19. A systemcomprising: a first NOx sensor positioned in an engine exhaust systemupstream of a selective catalytic reduction (SCR) catalyst; a second NOxsensor positioned downstream of the SCR; a first temperature sensoraligned with the first NOx sensor relative to an exhaust gas flow in theexhaust system for measuring a temperature of exhaust gasses flowingpast the first NOx sensor; a second temperature sensor aligned with thesecond NOx sensor relative to the exhaust gas flow in the exhaust systemfor measuring a temperature of exhaust gasses flowing past the secondNOx sensor; a controller in electrical communication with the first andsecond NOx sensor via a CAN bus, the controller having computer-readableinstructions for: determining that one or more of the first NOx sensorand second NOx sensor are degraded based on outputs from the first NOxand temperature sensor, and the second NOx and temperature sensors,respectively, where the outputs are generated during a self-diagnostictest after a vehicle ignition key-off.
 20. The system recited in claim19, wherein a test result, test status, and one or more NOxconcentrations and oxygen concentrations of exhaust gasses sampled bythe NOx sensors are estimated based on the outputs received from thefirst and second NOx sensors, and where a temperature of exhaust gasesis estimated based on outputs received from the temperature sensors.